I have a Bubble Magus BM100 that does the exact same thing. Cruising along for a month with no issues, skimming nice dark crud, then BAHM it goes nuts and skims GALLONS of water in a few hours. It used to cause havoc with salinity and chemistry because i would not always have salt water mixed at temp ready to replace the 2-3 gallons it pumped out into the 5 gallon bucket that my skimmer cup drains into. I havent figured out what causes it to suddenly skim like mad, BUT i have figured a work around to stop it. My skimmer cup drains into a 5 gallon bucket, I have a dual float switch in the bucket and in the event the skimmer goes mad, after about 1 gallon of over skim the float switch turns OFF my skimmer and i can turn it back on and tweak it when i get home from work. So.. i didnt answer your question, but you may be able to use my idea to help eliminate any crud overflowing back in your sump. One lesson I have learned in this hobby, float switches are your friends, have them EVERYWHERE.