Yamaha YG6600DE runs fine for about two minutes, then dies, as if switched off. Will restart after a few seconds, but runs for about a minute, then dies again. I know it's not a fuel/carb issue because about 8 years ago, I converted the unit to dual fuel propane/gasoline setup. When connected to propane source, it acts the same way. Oil is full, and this happens with no load on the generator. Ideas?