I have lots of problems with backfeeding when threshing downed wheat. Seems the long straw goes through in bunches, even leaving little bunches like "camel backs" in the unspread windrows behind the machine. The engine is constantly surging as the bunches go through. I'm using Marlins impellers. Could this be the problem? My feeder chain and drum are adjusted as per specs. 2188. Any suggestions? Thanks!