I might be way off the track here, bit i seem to remember helping a bloke with the same issue a few years ago, and i think the problem might have been dry solder joints on a relay on the mpu board....all i remember it was a relay on the right hand side of the backbox.

I remember you having to teach me the difference between a GI relay and a flipper relay. 

That was back in September 2010.

Thanks Rob.