I posted on this a few weeks ago without a real answer...I swapped a 91 n/a engine into a 90 n/a. The car now idles normal upon starting, but after driving, the idle surges from 1000 rpm's to 0, until it will finally stall out. Never had a problem with engine before, and it doesn't make sense to me...just swapped the engine from 1 car to the other without adjusting anything.
Any ideas?

 

I Had the same problem with my turbo II when i rebuilt the motor. Check all your vacum lines for leaks or if any are connected to the wrong places

 

OK, this clue should help me....when I run the car with the A/C on, the car idles perfectly at 700 rpm's. When the A/C is not running, the idle surges until it stalls out.
Now does this sound familiar??
HELP- I don't want to always run the A/C...

 

the a/c is just upping the idle...check for vacuum leaks! it could be leaking anywhere, im pretty sure thats your problem...