Pontiac Bonneville stalling

I have a 97 Pontiac Bonneville SSE. When I start the car in the morning and drive to school everything is normal but as I get onto the highway in rush hour and have to slow down the car stalls, but I put it in neutral and can usually start it back up and not have a problem throughout the rest of the trip to school. Then in the evenning when I start the car to go back home, I get on the highway ( w/ no rush hour) everything is fine up untill I get off the highway and have to either slow down or at full stop the car stalls but I can start it back up agian as before. It only happens during these times. I dont know if this is important but a couple of times when im driving at a slow speed the car seems like it has a spurt of gas injecting and the car speeds a little. I dont know if this is a coincidence but 2 weeks ago I had the alternator and belt replaced and then 2 days later experienced these problems. I cant afford much on loans, Otherwise I love the car, but what do you think is the problem? I searched online for same types of problems and from what ive researched it might be the "crank sensor" or fuel pump. Is this a dangerous problem? :( Expensive problem? Please help! Thank you