My GT6000 runs great most the time, but every once in a while it starts running rough, coughing badly, blowing bluish smoke. It only happens under full power while mowing. I've changed the gas filter and put in new plugs. I usually dis-engage the blade, slowing the throttle or not does not change the way it's running. It usually last for bout 5-6 seconds then starts running good again. I'd say it only does it once every 2-3 hours (I mow a big yard), and usually when I'm on a slight side slope. I run a tank with sea foam in it couple times a year to keep the fuel system clean. It has little over 200 hours on it. 26 horse eng.