Put a straight edge on the alternator pulley. My bet is that its centerline is off in at least one plane. Or the pulley centerlines are forward or back from one another. The straightedge should stay perfectly square to both the crank and and water pump when placed on the alternator pulley. If the straightedge doesnt touch both faces of all three pulleys you have an issue. Do the same with the crank and water pump. I've had combos throw belts. Every time it's been one of 2 issues. Alignment or bracket flex. Currently have 3 cars. All have modified brackets, pulleys, etc. None of them ever through a belt. All run to at least 7000.
Doug
and will add
the longer the distance is between the pulleys and the less contact around the pulley the more prone they will be to throwing the belt.
Some of the c-2, c-3 corvettes used an adjustable belt guard that would not allow the belt to raise up enough to come out of it's groove. This was placed on the adjusting arm between the water pump near the alternator. NOTE: the alternator is the smallest pulley with the least amount of contact. This was on cars that had L (drivers side) mounted alternators.
Another trick from years ago was to place a long 5/16 or 3/8 bolt just above the belt to limit its upward movement thereby kepping the belt in the pulleys. Again this was done between the alternator and water pump.
Interesting that several have commented to Dayco top cog belts have resole their problem. I do know there are some slight variations on the belt V angles being made by different manufacturers. That may be the root cause
