Not a pretty sight when you fill up the fan is it? In the grain tank and getting the fan cleaned out.
You didn't say what your sieves are set at.....presieve, top sieve and bottom sieve? The sieve settings are most likely the cause of your problem. When I've had this happen it has been from running my bottom sieve too tight. Always run with the presieve tighter than the top sieve. In soybeans I usually have it at notch 2.