It is switching off prematurely when a certain internal temperature is reached. As the internal temperature drops then conditions are reached that enable it to come on again.
I just guess.
A bizarre bad connection issue with internal wiring or the control electronics? Some problem with the fan/air pressure switch combination - being "on-the-edge" - good enough to work at lower temperatures but not when warmer.
What is the fan doing when it shuts off?
What is the Air Pressure Switch doing?
Peccavi
January 2013
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