when i have my bushhog attached and running, my ford jubilee will run for about 30-45 minutes then it begins to lunge and buck. the longer it does this the closer it gets to stalling and will eventually stall if not stopped. after it stalls, it starts right up with no trouble. while it's doing this, i can press the clutch pedal down and it will stop lunging and bucking and begin running perfectly. this does not happen every time i use my tractor. the fuel tank is over half full, the engine is not missing. what is causing this? could it be the governor has too much play in it and is moving back and forth excessively when i'm bushhogging? please help.