ive got an os 50 doin exactly the same thing. runs great heaps of smoke ,cool , just has this habit of cutting in and out ad therefore making tha tail tiwtch mine is especially bad when i powerclimb.. i thought it could be a lean spot. maybe some crap in the carb or fuel filter ,, ive ripped out the gine tightened all trhe bolts cleaned the crab the fuel filters so heres hoping ,thogh i have my doubts . im gonna fire it up this afternoon so fingers crossed .