The "ran for twenty min. then quits", brings to mind a repair bulletin, about gas tank filler caps vent plug blocking up and causing fuel stoppage, because of negative pressure in the gas tank. try loosening the cap after it stalls, and A) listen for a slight hiss of air, or the plastic tank slightly changing shape, or B) clean the cap, or buy a new cap. It's a long shot but sometimes it's the simple things. good luck