It sounds like the CPU-board. I would have started with a reseat of all the socketed IC-chips on the CPU board, including the ASIC (Use the special tool for it.). I never swapped CPU-boards without changing the game-ROM and the security chip, but it's only two chip to move. Get yourself a chip-lifting screwdriver, if you don't have one. Works very good, and is very handy when doing reseat on IC-chips.
Chip lifter (resized).png