I have a '99 GS3 w/ 105,000. I am fully aware of the low idle problem, but have read that it is most problematic with the A/C off. However, I have a low idle (around 400RPM) when the A/C is ON. If the A/C is off and I start the car, it idles fine. If the car is idling w/ the A/C on and I then turn the A/C off, RPM's shoot up to 700 or so and stay. Anyone know why I'm having this problem?

 

Sounds like your alternator isn't kicking out enough amps. May wanna get it checked at autozone or pep boys for free. Just replaced mine about 3 months ago.

 

Maybe your idle control valve is on its way out.

 

There is also a thread somewhere around here about adjusting the throttle cable. But before you do that definitely get the alternator checked out.