The march pump works fine at lower temperatures but during three recent brew sessoins the pump feeding sparge water at 180 (high for heat loss) stops pumping after a few seconds. The first time this happened, I took the pump apart and a piece of grain came out. OK, the impeller got stuck. The second time, there was nothing in the impeller housing. The third time I did nothing but ran a cleaning solution through it at teh end of the brew and it worked fine at 110. Any thoughts?