So I've got this solid state Wurlitzer electric piano that has started to distort. It's a model 200 which recently had the amp re-built (not by me).

The problem started when I accidentally left the piano on overnight. The next day, I turned it on only to discover a horrible distortion coming through the speakers (and the aux out) when notes were played. There was also considerable hum coming through the speakers that did not change volume with the volume knob (and the hum was almost in tune with G). Also, the volume of the notes was very low even when the volume knob was turned all the way up. Funny thing is, when I switched it off and left it for a good few hours, everything went back to normal - distortion and hum was gone and volume was back. So, now this happens every time I leave it on for too long (many hours).

Does anyone know what this might be? Bad transformer? Bad resistor? Or perhaps it is normal for 200's to do this when left on overnight?

The distortion is definitely NOT bits of dirt/fluff between pickup and reeds. I've had that plenty of times before and I know the sound.