on the older combines, there is a canvas hanging down from the top inside above the straw walkers, check for worn out or torn or missing canvas, replace that if needed, for some reason this has something to do with the air flow inside the combine, mine was torn but still intact, i replaced it and the same problem you are having i was having, this fixed it. dunno howIJIJIJIJ but i was told by an older ac mechanic about this...it worked!