It could be that the PIA message is a red herring. If the code gets lost due to any error on the address, data, or interrupt lines, it might "fall" back into executing the code to display the error message you are seeing. I would make sure to ohm out ALL of the data, address, and interrupt lines, and then ohm them out to make sure their is not a short between ANY of them... I know, what a pain.
Looking at all the lines with a scope can sometimes show a point where collisions happen if there is a short. Opens are harder to see that way. If someone else worked on the board, my bet would be that there is a hidden open to one of the EPROMS under a socket that is floating.
Mac