Idle Speed Wandering

I have a 92 C2 with 37,000 miles. I'm taking the car in next week to have an idle speed problem checked out, but I was wondering if anyone had any input before I leave it at the shop.

Here's the problem: Recently, the idle speed will wander almost to the point of stalling (about 500 rpm) and then surge to about 1500 rpm, and continue to wander up and down. BUT this only happens after I start the engine after it's warm, and the problem goes away after I drive the car for a few minutes.

In other words, the idle speed is fine when the engine is cold or hasn't been driven for a few hours. But if the car has been driven for a while, stopped, and then restarted within an hour, the idle speed wanders between 500 and 1500 rpm. The problem goes away once the car is driven for a few minutes. Also, when the rpm's drop to 500, the lights will dim.

I suspect a faulty oxygen sensor. Anyone run across this problem, in particular with a 964? Thanks a million for any suggestions.

First look for any loose electrical connections to any sensors. Second check for any loose vacuum hoses. Third if necessary take a read out from your computer. It's easy to do and for the cost of the Haynes Techbook "Computer Codes",available at most auto stores,its worth it's weight in gold. The Techbook cost about $12. It tells you how get the DME to spit out codes as a sort of morse code on the check engine light. Then you just cross reference the codes in the book to find out where the problem is. Takes the guess work out of it and lets the computer tell you where the problem is. I use it for all my cars with computers in them. It's great!
Good Luck Bobby 72'TE
PS my guess is a temperature sensor or an idle speed sensor.

[This message has been edited by Bobboloo (edited 10-09-2000).]