So you are cycling the audits and it hangs every time when you advance past audit #5? Try zeroing the audit 5 (use the button on the top middle of the MPU to zero an audit, sometimes a coin switch works) before stepping forward. Try a new 5101 RAM.

i would scope / probe the pins of the 5101 like OE, CE1, RW pins of the 5101 to see if they are all pulsing. CE2 should be high (reset).

swapping out new u7 and u9 is pretty painless if you have other known goods.

probably not the button.

someone just had a post where a R/W input to the 5101 on williams game caused similar sounding issues.