Does it do the dimming/shorting thing if you simply manually start the score motor running? If it does you could pull the motor disconnect plug and slowly turn it by hand and see exactly where in the cycle and which switch stack/circuit it may be shorting at. If it's not that, could try isolating exactly where it happens by activating playfield switches one by one. The motor stacks and bottom relays are a possible place for more solder blobs to have dropped onto and shorted something.