Here's a bump for you - I have no personal knowledge about this but I found a couple old rgp posts that make it sound like this error can be caused when the ROM versions don't match between the CPU and the Display...if yours haven't changed then maybe you should try reseating the ROMs in their sockets?
Good luck!
http://newsgroups.derkeiler.com/Archive/Rec/rec.games.pinball/2005-07/msg00888.html