intermittent idle issues, not ISV

[David.Ullrich at ferguson.com]

OK, I've got a 1987.5 Coupe GT (NG engine). I've got an intermittent idle problem. It always starts fine (cold or warm) but after idling for a minute or so it may start bouncing from 900 - 1600 (can't be exact as I have a digital dash). It never falls below 900 or so. It slowly raises up to 1600 then dumps back down. I know the ISV is working properly and is clean. When removed, it opens and closes smoothly and quickly when applying 12 volts.  Oh, the idle issue doesn't always happen and if it is happening I can just turn the car off then restart to cure it for a while. Any thoughts? Excuse the amateur question...I'm new to this car...

Dave
1987 Coupe GT Special Build 2.3
1998 VW Passat mit UPsolute (for SALE, CHEAP)