There's no idle control per se on this car, idle is controlled by the electronic throttle valve.

If it runs fine under load, it is unlikely you have wiring harness problems. You can sometimes determine the age of the harness by locating the part number tag. It is near the ECU, in the electronics compartment behind the battery. Remove the plastic shield to expose the ECU, ABS contrller, and OVP relay.

For this car, step 1 in diagnostics is to build the code reader. Search for the "Arthur Dalton code reader" for the parts list at Radio Shack - about 10 bucks total. Pull the error codes, clear the error codes, drive it, and see what comes back. That's the starting point.

Could be a vacuum leak, though this engine control system can compensate for vacuum leaks, to a degree. The plastic intake manifold has been known to leak, and there are some vacuum lines underneath the intake.

I've seen fluctuating idle but otherwise good running on this engine from an old O2 sensor. How does it run when in open loop (e.g. from a cold start)?