If the PO had a couple break on him that makes me think something's out of whack. When you retension those there's a possibility that it's not lined up correctly? Sometimes if they're not lined up right then one side of the pulley will chew on the belts, so that might be something to check. Seems to me like belts are NOT something you should need to replace every 100 hours. Definately check for lots of black dust/soot, whatever you want to call it. that would be a good indicator.