My sons 89 model r32 had the same problem found it to be the fast idle vacuum, its under the throttle body just to the back of it .If you follow the hose that runs m your crossover pipe s the hose that runs at 90degrees from it to under the throttle body 2 bolts hold it onand an electrical connection .
Once you have it off there's 4 screws hold the backing plate on ,you have to take it of and once you have you will see a black round disk that has small amounts of carbon build up on it you have to clean this of with carbie cleaner was the best i found to do it .there is a samll return spring in there be carfull not to lose this as it the return spring for the fast idle not very strong the carbon build up on the plate wont allow this spring to turn fast idle off returns it slow if at all .
I found the cure on this forum if you can find it he has putures there to help you
Cheers Peter