Mitsubishi Repair: Irregular Idle, exhaust gas recirculation, mitsubishi galant

Question
1992 Mitsubishi Galant ls. 2.0 SOHC, automatic
I am experiencing problems with the way this car idles. When it is in park, there is a 200-300RPM fluxuation and the throttle itself is moving. When the tranny is in drive and I am accelerating there seems to not be a problem. When I let off the accelerator, the throttle acts up again. When the car is stopped and in drive, it wants to move forward so much that the brakes barely hold it  back. So far I have had the timing and balancing belts replaced and adjusted, the o2 sensor replaced, new fuel and air filter & new plug wires( plugs and cap seem ok). Since I could not locate new OEM throttle body components (sensors and motors) I had to use ones from the scrap yard which were said to be functional (right...). Replacing the throttle body components seemed to help for a second or two but then it started to act up again, leading me to think this is a problem arising from another malfunction down the line. I'm narrowing down the possibilities, I still haven't checked the fuel pump but I assume it works because the car is ok when accelerating. If you could give me some suggestions on where to look next I would appreciate it. Also, do you know of a good source to find reliable parts for this thing?
Thanks, Seth

Answer
Seth,
Your problem could be related to the Idle Speed Controller (ISC) or your Exhaust Gas Recirculation (EGR) valve.  You may have replaced the ISC and/or EGR when you replaced the throttle body components, but it's very possible those components were not functioning either.  If you have a service manual, you should definitely check the ISC you have on your car.

But it's possible your problem may lie in your car's computer.  Read this article for more information:
http://www.90gsx.com/eclipse/cap/caps.htm

If the ECU is malfunctioning, you'll get all sorts of strange problems.  It's a good idea to pull the ECU and visually check it out.
Good luck!