'94 suburban, 5.7L, 2WD.
Belt came off Thursday last week while wife driving. I went and put the belt back on, it was pretty tore up so I went straight to auto parts store and put on a new one before driving home.
Next day, new belt came off again while she was driving. I went and traded cars with her and drove the sub back to work and later home that evening.

Belt never came off with me driving, but I didn't run the A/C. So I started it in the garage put on the A/C and when I revved the engine (by hand at the TB) I could see the belt skirt across the idler pulley (located directly beneath the A/C compressor).

Got my wife to sit in the truck and turn the steering wheel with the A/C running and the belt skirted across the pulley.

Any ideas?
Has the tensioner pulley just started to wear out and the load on the belt from the A/C and turning making it fail?
By the way, I replaced the idler pulley (directly under the A/C) but it didn't make a differnce.