I had the same problem with my belt happend twice in about 3000 miles, tore off one groove, it ended up being the Idler and water pump pulley, changed both and has been fine for 30000 now, they did not feel that worn, but just enough between both to cause the belt to run off.

 

Mine started squeaking a little bit (more of a chirp) so I got a brand new NAPA belt. It was fine for a day but the squeal came back twice as bad. I replaced the tensioner and it did not help anything. In desperation I ordered a Goodyear Gator back and the noise has totally gone. It's a great belt because of the design of the ribs. It's nothing like the stock belt or the NAPA unit. It also has a lifetime warranty...

 

Come to think of it my squeal started about the time i put a Napa belt on my truck.