exact same problem here with a wright sentar . i would mow for about 15 minutes and just the blade would shut down . VERY aggravating. i replaced the PTO switch and the relay and still no go. turns out that the plug that connects the wiring harness to the clutch was laying against the back of the engine block and the little clip that holds the connectors together had melted just enough that any vibration or bouncing around was disconnecting the PTO at the plug. i ty-wrapped the plugs together and re-routed my wiring and have had no problems since.