Check the bearing in the idler pulley. It will make it bounce if it is a little rough. Hard to detect sometime. I'd replace it before I try to change or add more tension.
easyrider
High load causes the belts to hop on blowers and turbo's as well and when the harmonics couple between the crank pulley and driven pulley up they can jump right off. (He did say it was on the dyno)
Stopping that harmonic effect by interrupting it works wonders without putting more load on everything else with bearings in it.
