2002 Buick Rendezvous serpentine belt keeps sliding off and

I have had two mechanics look at the same problem 5 times in the last two weeks. The first mechanic just redid the upper part of my engine. I drove it a couple of days and the belt came off. I recognized the problem because this is what happened to it before the engine went bad. I raised the hood and pulled the belt out; not broken but frayed from riding on the edge of the pulley I guess. So the same mechanic came and put another belt on and I drove it back to his shop and made it there. I picked it up the next day. He said that he had fixed it and that the pulley(tensioner?) Had two spacers which was making it out-of-line with the rest so he took one out. It broke another belt. I took it back. He said that he thought someone had put the wrong pulley on the car at some point(tensioner?). So he replaced this pulley and put on another belt. The belt broke this time a few days later when I was 3.5 hours away. So a different mechanic looked at it and said I had the wrong pulley on it (tensioner?) And replaced it with the size that was on it before. He also showed us that the pulley attached to the power steering had broken in the middle a little bit and then replaced it as well and put on another belt. I drove it 30 mins. Away. Today I got in my car thinking maybe it was finally fixed. When I cranked it, I heard the sqeaking again, looked, and the new belt has moved over the raised edge of the larger pulley attached to the power steering-again. (It seems to be squeaking from rubbing against the power steering fluid tank.)I decided to cancell my appointment and not drive it. I need to know why I might be having this problem. I noticed that a few timessince I got my car back from the first mechanic(not every time and not often) my car hasn't wanted to fully crank up and it has taken me several tries. This may or may not be related, I don't know a lot about cars. I mentioned it to the mechanic yesterday and he said he didn't think it was related. However, I have read a lot this morning and thought I was still worth mentioning here. Please help. I need to make it to job interviews!