I tried the suggestions regarding vent line and removing the gas cap, unfortunately it didn't make any difference. There must be an answer to this. This is my third season with this tractor and it runs great until maybe 20 minutes or so and it will sputter and eventually shut down. I can wait about 5 minutes or so, turn the key and carry on until it sputters again and stops. I mulch using the mulch plug and have always done so. I sure someone will have the answer. Thanks!