I have an '89 Formula 350. Sometimes the idle becomes too high when in park 1200-1500 sometimes a little more and while driving lets say at about 55 mph the rpm's are at about 2500 which i know is not right but this doesnt happen everytime. When i slow down and come to a stop the car's rpms then go below 500 and wants to kill but then comes back up and most of the time sits over 1000-1500. I've cleaned the TB and most of everything associated with it. Any suggestions on what I can further check.

 

sorry to bump this but my 91 V6 does this too. Anyone?