I have a Murray Select model 465306x8 that while i was mowing after about 30 to 40 min it started to sputter and almost die while the blades were on i then turned off the blades and the engine sounded strong no sputter or miss could be heard but once engaged the blades again back to the same problem...let sit for about 20 min and then ran fine for another 30 to 40 min im at a loss i checked the fuel filter that looked fine but the air filter was pretty dirty but could that cause that much problems im at a loss here and don't know what could cause it any ideas???