1991 chevorlet Corsica lt stalls at stops and low power

My 1991 chevorlet corsica will start and run in park with a little low power and may stall when I put it in reverse AND or stall at a stop sign. I then got to put it in park and start it several times and give it gas from the pedal then it finally drives ok but it hates to stop. It wants to cut out but if I put the pedal to the floor it may get enough power and continue on. It runs great on the highway and never cuts out while im driving. After I get off the highway and stop at a red light it seems to be ok but this is after driving it for a while. I will drive it around for a few minutes and let it pull its stalling issues and then drive somewhere because it seems to stop.I have already tried a fuel system cleaner that claims to stop loss of power and clean the fuel system and all that good stuff. Anybody got any idea whats causing my car to stall and what new part I may need and how expensive it is this part IF a part I need? My e-mail is Messerschmitt387@aol. Com because I aint the best at finding sights I left questions on. Thank you all for your help !