Developed problems at about 70 hours (home mower). Have taken it to the dealership 4 times but no joy. Mower shuts down (cuts out first) after around 20 minutes if engine is runs after sitting for several days. Will restart and run for 5-10 minutes after cool down of several minutes. Dealer replaced fuel shutoff solenoid, then replaced it with a bolt, then replaced fuel filter, then fuel pump. None of this has worked. I'm thinking it is in the ignition rather than the fuel system (dealer states that they "went through" the carb). What do you think?