I just cleaned my IAC solenoid in my 3kGT this morning... and was disheartened to find that it wasn't the cause of the problem.
If you're having the fluctuating idle, the first thing you want to do is pressure test your intake to make sure that there aren't any leaks. Also, remember that you want to WARM THE CAR UP before you test it. The way the idle control system works in most cars (3kGT and EGT included) is that there are TWO valves that bypass the throttle plate. The frist is the IAC solenoid, which is controlled by the ECU, and the second is the bypass air valve, which, on the 3kGT, is a bimetallic valve that expands and contracts with temperature. So when it's cold, the thing is fully retacted and air passes through, but when the engine warms up and the coolant flows by it, the valve shuts itself so no more air passes through. That is why, even though your IAC solenoid may be dead, you car will still idle when it is cold, but will not when it is at operating temp. That's also why you sometimes see coolant lines running to the TB of some cars. It goes to the BAV.
The first thing you want to do to check the IAC is to check the resistance through the coils in it. I'm not sure what the specs are, you may want to consult the haynes manual. Chances are that they won't have it because the haynes manual sucks. You may have to look at some in depth technician's manual or somehting.
I've seen a bunch of different things mess with idle. Sometimes it's the fuel filter clogging up, sometimes its a vac leak. I've even seen somebody's crank angle sensor mess it up (which will throw a code).
The first thing I'd do if I were you it to take it to an autozone or something and see if you're getting any codes thrown.
Next, I'd get the engine up to operating temp and see what your A/F ratio is on the O2 sensor. You may just be running really lean or rich.
If you've got any friends in the area with EGTs, ask them if you can swap their IAC in for a sec to try it out and see if it changes anything. That way you can rule it out if it's the IAC.