has this been going on dince day one? GM (and some Ford) had a bad
problem with this. due to the fact that they used two different sizes
of pulleys on the crank, yet used looped the two belts over the water
pump. One two the Alt, the other to the Power steering. the PS is a 1/2
in belt, and the alt. is a 3/8. That's standard, but on the harmonic
balancer the two different size pulleys would naturally make a
grotesque mismatch, and one belt will win, the other will lose. and it
just so happens to be the smaller alt. belt.

See if the lower pulleys are mismatched.