Mower starts and runs then begins to die out. I replaced the air and fuel filters and plugs. The best way I can describe it woud be to say the throttle is going from fast to slow. The mower usually dies when mowing, is restarted and jockyed back to the trailer. Equally as important, the problem is only obvious about 60% of the time meaning there are entire lawns we get through with no issues. We had a mechanic look at it last year, he made some adjustments to the valves and cleaned the carb. Of coure it worked fine for him/in front of him/on his shop lawn. We took it to our frst lawn and wammo, problem! Any clues?