got belt slippage, either rotor belt or belts driving rotor, check tension on belts that drive rotor, check pulleys for wear, don't have book handy, ops manual tells ya measurement on rotor shieves, might have to take cam assy apart, possible binding due to wear, i'm guessing rotor slow down poor sample, is this something that just started? bet if you put both of sets of belt on life will be good, scott.