My car shut off while driving, What should I do?

I have a 1995 Pontiac Grand Prix GTP 3.4 liter v-6 and it shut off on me while i was driving. Ok so here is exactly what happened. When i bought the car it did the same thing it did now shut off, so i took it to a garage and had it looked at. well the garage i took it to didnt do **** but put a battery in it. So the mechanic said we cud come pick it up everything was good to go. well we drove it about 100 miles and it started spazzin. the car jerked and all the service lights on my dash came on and my power steering, power brakes and gas pedal all stopped working, and then the car shut off bout 5 seconds later. Now i am pretty sure that its an alternator problem and it drained the battery due to a bad alternator becuz when i try to start it, it does nuthing. I am in desperate need of knowing what is wrong with my car becuz i dont want to take it to another garage not knowing what needs to be fixed. Please help anyone have an idea please answer, thanx a million to those who answer.