#3 is your proof that this is NOT coming from the power company.

Two things come to mind immediately, in order of likelihood:

1. There is a problem with your Neutral connection to the new panel. Most likely the new and old panels are sharing a neutral to the utility pole. If your new main service was installed first, then the neutral for the second one is likely on a feed-through lug off of the first one. That would not have been the original lug, so most likely someone had to add it and in doing so, failed to fully tighten the connection to your neutral bus for the first service. So at night, when you are likely using your PCs, TVs etc. that have a higher harmonic effect on the neutral, the connection heats up and you lose it. With everything off, it cools down, you get a connection again, and start over. During the day, the loads on the neutral are less and/or there are less harmonics so it doesn't happen.
2. Gremlins...
   

Sounds like a loose/corroded connection...If you have access to the connections at night you can probably see the arcing at the bad connection....

I wired a couple of houses in a previous life, but since retiring and getting a lot older, a lot of not recently used info is getting automatically deleted from my hard drive. Please forgive my present state of ignorance which prompts these questions.

1. I'm assuming you mean that in the evening, after removing the breaker box panel cover exposing the breakers and panel connections, I am looking for an arc at or near the neutral buss?

2. Earth grounding of breaker panels, (by copper rod) in parts of Mexico is not as regulated or reliable as it could be. Could this be part of the problem?

3. Could the bad connection be where the Power Company tied the new ground into the previously existing one? This was done by making several wraps around the unshielded ground wire.

4. The old service circuit is still intact and works perfectly. The new one was created about 2 weeks ago and hasn't had time to become corroded. Therefore I'm probably safe in assuming it's a connection problem?

The problem could be on the hot side or neutral either one, where a connection has been altered or added would be the most likely location...The copper rod grounding of the panel is more for safety than being part of the circuit...Yes at night or in darkness you should be able to see the flashing of the arc of an intermittent connection, in fact you may be able to actually hear it as well...The wrapping of a wire around an existing unprepared wire, sounds like a possible location to check...

Assuming, from my experiences, that the residential power delivery systems in in your part of Mexico are like they are in the US, it would not likely be on the line side because it would need to happen on TWO connections for the entire house to go dark, otherwise only 1/2 of the circuits would. That's why I concluded it was a neutral connection issue.

I agree with your logic, but I'm confronted with a confounded variable. Essentially what happens is that the mini-split air conditioner in our bedroom and all the lights in the room go out. I can tell that some patio lights are still on, but don't have time to check everything since the outage is so short-lived.

I've started to map the circuits and am about convinced that all the circuits in the new panel are going off. Before that, I was considering that one phase of the hot side had been interrupted and the lights which remained lit might be powered by the other phase of the same panel.

Until I get it totally mapped, I'll continue to have some small degree of uncertainty.

The electrician who installed the new panel from the meter came today and checked all the neutral connections throughout the system. In addition, he added a neutral buss jumper between the panels. If that doesn't fix it, I guess when it next occurs I'll be running to the garage in my PJs to take off the panel cover to look and listen for arcs.

I wanted to thank you, JRaef< for your original response, but wasn't sure about the forum protocol. I sincerely appreciate all the ideas you guys have given me and I'll try to post another response once the issue seems solved.

Thanks loads

Due to your "shared" neutral inside your service entrance panel and the fact there are multiple pieces of equipment plugged into the outlets and/or hardwired, you can/will experience "feed-through" between devices/equipment which will allow limited operation of some "stuff" depending on current draw and impedance especially if the issue is with the neutral.

Be careful as this can mean that the neutral is "hot" and therefore an electrocution hazard.

Measure your voltage in both feeders in day time and night and speak to your neighbours too. Tell the utility to check whether there is an intermittent voltage drop/fault in any phase in the night when transformer is fully/over loaded?. For a day(24hr period)hire a datalogger for each feeder and record the V,I,PF etc and show it to the Utility.

Since both my meters and respective panels have the same feed from just before the service entries, if the problem was one of time-of-day voltage drop or fault in any phase, would it be possible for only one to shut off and the other stay one?

In addition, last night was the first night we had a different pattern of outage. A few minutes after retiring to our bedroom, turning on the air conditioner, a couple of lights, computer and printer, everything shut off for about 45 seconds before coming on again. Once again I could see it was only one of my panels that was affected because my outside lights, powered by the other panel, stayed on, and all the neighbors I could see at the moment still had power.

If connections are the problem, the only places I can think of that haven't been checked are the connections to the main breaker, connections in the KwH meter, and the splices made by the Utility. In addition could the problem be either a faulty main breaker or KwH meter?

I'm actually beginning to consider the possibility of JRaef's suggested Gremlins if not Poltergeists.

Time to look at the meter, it's the only thing that's different about both circuits. It could be poor connections in the meter pan jaws which no amount of tightening will correct. If it's the old spinning disk type then there may be an intermittent in the current coil connections, if it's a digital type then it could be reacting to a remotely sent disconnect signal or interference from a local source of RF noise, or simply a defective meter. Time to call the utility in either case.

I have found many times when this scenario is present that the issue is with improper crimping of the conductor sleeves where they are spliced.

This is especially prevalent with aluminum wiring.

Any movement of the conductors will result in a loss of voltage whether due to heating/cooling or wind.

Have the power company replace the crimp sleeves and most likely your problem will go away.

My first apartment was an efficiency in half of a duplex in Philadelphia. Shortly after I moved in, I started having mysterious outages. None of the other apartments (there were 4) had the same issue.

It turned out to be a loose connection between the mains and my meter. Thinking back, I probably should have asked the tech to check the others in the building. There's no telling what caused it to come loose after who knows how long.

This is the next chapter in the continuing saga. Day before yesterday, the electrician removed the utilities conductor splice sleeves, twist-spliced the ends together and insulated them with good old black electrical tape. He then removed the meter and installed his own jumpers. For the first time in several days, there were no outages during the night. Yesterday, he replaced the meter. Last night there were a total of 8 outages, beginning an hour earlier than usual. His reasonable conclusion, giving all the other causes that seem to have eliminated, is that the meter is probably defective. He will call the utility company and he said they will normally have it replaced within 2 or 3 days. So far so good. With a bit of luck, a new meter may solve the problem.

My one remain remaining fear is that the cause is one of some sort of interference and since my newly installed meter is the only digital meter in this area, the utility may not have experience in addressing such issues. We'll cross that bridge (pun intended) when we get to it.

At any rate, I sure appreciate all the ideas and cautionary notes. I'll be sure to post another comment after the new meter has been installed. Thanks again, PataSalada