Instead of getting into game mode, can you get into test mode? If so, maybe you can cycle through the various tests to see if there's a specific component that's bogging it down, drawing too much current, shorted, etc. Another thing to try is to remove the flipper and solenoid coils.and try starting a game. If it let's you start a game then it may prove that there in fact is an issue of a coil choking out the MPU.