I have just had a close study of the Shrek schematics and after my brief look I suspect it has something to do with the zero crossing detector circuit.

Now the problem here is that the entire machine relies on 'knowing' when the AC waveform 'crosses zero' and the processor uses this information to dim lamps smoothly, fire solenoids for an exact time, clear lamps and probably zillions of other things I haven't thought.

A very complex fix I'm afraid and probably could only be fixed by replacing the entire SAM MPU board which would probably be an expensive exercise?///