Do you have the circuit diagram? Can you separate the pre- from power-amp parts? It sounds like you have a malfunction that may be after the pre- but is either in the power amp circuit board or after. I don't know what to make of the fact that it works briefly after a reset then cuts out. If this is a class-D amp, I wonder if there is some problem in the power-amp, but I don't have a clue as to how you'd diagnose it!