Sounds to me like there may be more corrosion at play here. If there was no history of corrosion, I'd say that the issue could be the sockets - CPU, ROM and 6464, or the ASIC is loose. With the game on the bench, you need +5V but I always have +12V as I simply use an old PC AT power supply. With the CPU PCB on the test bench, see if you can get it to fail - push the ASIC in and the socketed chips until it locks up. If it does not, then it will be something on the driver board, or the ribbon cables.
Have you got another machine to swap out ribbon cables ?