If you throw a belt suddenly, it is either a bad tensioner or stretched belt. Seeing that your car is a 93, im guessing that the tensioner is going bad.
Sometimes if you replace an accessory drive (alt, PS pump, etc) the new pully might not be inline and the belt can walk off the pulley, but check the tensioner.
Most tensioners have a arrow on them and three marks. When you put the belt back on look at the tensioner and check of there is an arrow. If there is then see that is is within the two outter marks. Then have a friend with the car in park and ebrake fully engauged rev the engine. Watch to see if the arrow stays within the two outter marks. if it goes outside the tensioner is not keeping proper tension on the belt.
When the car is off the arrow should be some where in the middle of the two marks. Remember to keeps hands and fingers out of the belt drive when the car is running. And be ready to duck incase the belt flies off durring the test.
Also, when installing the belt moving the tensioner should be a bitch and almost take all you have to hold it and route the belt. If it isnt that might be another sign that it is bad.
2005 Dodge Magnum R/T-Can you say...HEMI?
1999 Bonneville SSEi-Going Fast...With Class!
1989 WS6/L98 Trans Am-Got Torque?
1999 Grand Prix GT-Wife'* Widetrack
1995 Camaro Z28-Project Bitch