Hi, this is Russ, I'll be helping you with your problem today. THANKS! This is caused by trash in the carburetor. I would remove and disassemble the carb,soak it in carb cleaner overnight and rebuild with a carb kit from the local dealer. The engine is running very lean and the mechanical governor is trying to stabilize. A good soaking and carb kit is the answer here. Remember to DIAGRAM ALL LINKAGE AND PARTS SO REASSEMBLY GOES WELL.