Engine Revs on it's own when in idle

I own a 1995 SL1, 5 speed manual. I recently experienced my engine revving to about 2000 RPMs or more on its own when I put the car in neutral as I come to a stop or shifting gears. The "Service Engine Soon" light also comes on but when I move off and reach third and fourth gears it goes off. After driving for about twenty minutes the revving stopped and the car seemed to idle normally. Is this because the car was not warmed up properly or something?. This is the first time this has happened though. Has anyone else experienced this?