Whitfield Optima 3 Insert... 10+ years old. Only runs on low... Have cleaned throughly. Replaced Control Board, Thermal Discs, Door Gasket, Bipassed Pressure Switch. Both Fans working... Auger obviously works. Startup on High... 20 minutes of high heat producing flame, then dies out and shuts down... I've thrown the book at this darn thing. What am I missing???