So. I just got a 1986 GXL. I've gotten some big bugs worked out but now I'm having some idle issues.
A quick summary:
Yesterday afternoon - took a couple tries to start, when it got going it climbed up pretty steadily to 3000rpm. It slowed down a little but kept going up. It went back down when I pushed the clutch and brake pedal. It dropped down to 500ish and settled at 950ish but continued to waver.
Yesterday evening - someone came over to help me out, we started it up and it did about the same thing. We drove it around the neighborhood and when we got back I adjusted the throttle body so it was steady at 1100rpm.
This evening - Took a couple tries, started and went up 2000rpm. Took a complete nosedive and almost turned off then evened out at 1500. I watched it for a bit after that and it floated around +/- 200rpm.

So WTF??? Is some idle air control valve work in order? Suggestions?

 

Fix all vacuum leaks.

Adjust the TPS.

Check the operation of the BAC valve.