Sad about stalling

My 1992 Buick Regal Custom 3.1 liter with 100K+ miles has recently started stalling while stopped but in gear. The check engine light is not on and I'm pretty sure it works because it lights up when I start the car and when it stalls. The stalling seems to happen after making a more sudden stop or after being uphill or downhill although it can happen other times too. Furthermore, when I start up the engine sounds like it's revving itself, and I sometimes hear this while idling at a light as well. This sounds worst right around the time it's about to stall, which is happening about twice a week now. It drives perfectly, the transmission fluid was changed last year, and I had a fuel injection tune up recently with a throttle plate cleaning in an attempt to correct this problem. Any ideas as to what could be wrong? Thank you!