During mowing my engine often stumbles and begins to quit. Sometimes I can get it to resume operation by holding down the brake pedal as the engine emits a lot of smoke but gradually comes up to full speed with no more smoke. It seems that something is cutting off the ignition temporarily. When the engine does quit it is easily restarted as if nothing happened. I checked the seat safety switch and it seems to operate as it should. Are there any other safety switches that could cause this problem and where are they located? Thanks for any help that you can provide. Ray