My B8200HST starts and runs, but when it gets under a load cutting grass it gradually starts missing and starts losing power. It will run good for about 3-5 minutes before it starts doing this. If I shut the pto off and idle the throttle back quick enough and wait, it will smooth out and I can take off again only to repeat this senario in another 5 minutes. I have changed the fuel filter and air filter, bled the fuel lines and injectors, even ajusted the valves. No change. Any suggestions?