I have a Scag Tiger Cub mower with 60 hrs. Until recently I have had no problems; now it will "cut-out" and "stall" after 30 mins, and I have to re-start it. I have taken it to the dealer 3 times to no avail. They insist there is nothing wrong with it. The air filter is clean, the gas is fresh and the mower deck has no accumulated grass on it. If I stop mowing when it starts to "cut-out", wait 15 minutes it will run again for 30 minutes before the problem begins again. Any suggestions?