Try an idle relearn, my 1992 CS 3.1 did this very thing until I pulled the ECM fuse, put it back then started the car and put it in drive and let it idle in gear for 5 minutes.

My 93 GP did the same thing and it was fixed by putting in a junk yard PCM. The stealership scope said my car was fine too. Only thing is, I had to chase the problem. First TPS, then ign. module. Then on a hunch, the PCM. 120,000 miles later, no problems.

although this may be a possibility, id say check the cheap and easy stuff first. just inspect the vac lines. i would check the one that goes to the purge canister. they seem to rott out fastest. on the w body cars it goes from the intake to a steel line on the drivers side right by the brake lines.