Do you have any type of manuals? Try the troubleshooting guide.

Try on-line forums that deal with this specific type of vehicle.

Assumuing fuel injection?

More info.

On Fords its pretty common for the idle air control motor to go bad or just up and cause odd and intermittent idling issues.

I have replaced them at least once on every Ford I have had that was fuel injected. Usually they just have two three screws and an electrical connector holding them in so they are an easy fix.

Getting to them on the other hand.... on a V6 four cylinder I have no idea.

Yeah, if you aren't sure if it's a V6 or a four-banger, maybe you should leave this to the professionals :)

http://www.partsgeek.com/mmparts/idle_control_valve/mercury/topaz.html

Could be vacuum line ...
Repair manual...
http://www.2carpros.com/makes/mercury/topaz

I was going to suggest borrowing the OBD2 (on-board diagnostics, I think) tester from your local auto parts store to see if any diagnostics codes show up. However, being older than '96, your car doesn't have OBD2, it has a predecessor called EEC-IV. This site, http://www.corral.net/tech/maintenance/eecivtest.html, written for Mustangs but should apply to your Mercury, tells how to get diagnostics.

Here's a link to Technical Service Bulletins for your car: http://www.obd-codes.com/tsb/1993/mercury/topaz/ . I don't know if any apply to your problem.

sounds like same problem i had with a 97 mercury sable it turn out to be the MAF sensor on mine .

The idle solenoid is prone to gumming up with deposits. When that happens, the engine tends to stall when the engine is asked to idle. The computer monitors engine speed. When the speed is above the predetermined speed, the idle solenoid tries to slow the engine down. Because the engine speed is now being controlled by your foot on the gas pedal, the solenoid is allowed to go to the lowest position( sort of). But, when you take your foot of the gas the sticky solenoid valve will not react fast enough to prevent the stalling condition. As a test for this, get a can of spray carb and choke cleaner. Look at the picture provided in the above post, and remove the two bolts holding it to the engine. Spray the two passages in it with the cleaner, and reinstall. If that helps, or cures your problem, you know what the problem is. Cleaning it a few times will usually return one of these things to normal function. If the test cures your problem, and you choose to replace the solenoid, they are very common in the auto parts stores. Good luck.