Diesel Engine-Generator's (DEG) intermittent problem..

The problem is more than likely in the governor, which controls the fuel. You can start by checking the speed sensors that send signal to governor.. Remove the sensors and check for damage or debris.

Next confirm your actuator (the device that controls the fuel to generator) is not sticking..

Does the set start on Load .If so what type of loads?

All good advice,

I had a 320 KW Cat machine that would not hold load of load up to rated output.

After some trouble shooting and monitoring, we found that the fuel filter housing had been machined incorrectly at the factory. A major fuel restriction was the result of this.

With this defect the machine would start and run fine under small loads, but give it a big ( 75Kw) load increase and it would slow down to the point of underfreq. Reset everything and it would restart and run again under a reduced load.

After replacing the fuel filter housing the machine would pull right up to full rated power.

It sound like a fuel delivery problem to me.

One possible explanation for the problem you are experiencing may be found in the drive end of the Marathon generator. This equipment can not produce electricity without first having a small electrical current being applied to the windings. This is accomplished by the use of an "exciter" mounted in the end of the unit. In that exciter there is a small diode that may have failed or is about to fail. This diode provides the "excitation" required for the generator to produce the electrical current. It is quite easy to replace and may be the solution you seek.