Mine does the exact thing, VE equipped with JWT ECU. Didn't happen in the B15 but happens in the '99 G20. Every time the outside temp changes significantly I have to reset the idle air learning. Try running the motor till it's good and warm. Then unplug the TPS, the idle should fall to 750/800 and stay. Then when you plug the TPS back in the idle should level to what the computer wants it to be, which should be around 800. Sometimes I have to unplug the idle valve as well. I think that is a clue.
I cleaned the idle valve a few months ago and had no change. The A/C sets it back to normal idle almost every time. For me, mostly happens when the motor is cold but yesterday it happened warm and climbed from 800 to 1,800 sitting at a light. I reset the idle air and had no problems so far. I'm pretty sure it the computer doing it, based of what, not sure yet but the wire harness is pretty old and I suspect attenuation, especially after the regrounding you mentioned.
Last edited by phixer
on 2012-10-08
at 22-26-48.