Your problem just might be high frequency torsional vibration on the
back end of the crankshaft. I've seen this shred alternator/generator
drive belts on a C182 in just a few hours. Your alternator will still
work OK with this vibration (it will be about 250 Hz @ maybe 1 degree
p-p or so) but I'd be concerned that the cantilevered gear driving the
crankcase output shaft could break off from fatigue inside the engine
case and create instant and very severe trauma.

You won't feel this vibration because it is torsional, but the
resulting stresses in the propeller and crankshaft are very high.
Someone with a critical ear MAY be able to hear it.

How far is the engine from O'haul? Were the damper bushings and pins
replaced at the last overhaul?

I can't imagine the damper counterweights "sticking". The next
failure could be a propeller blade from dynamic stress. If this new
belt gives trouble, get someone intimately familiar with that engine
involved.

Good luck!