The reset is caused by a watchdog circuit. The watchdog circuit is probably on the other side of the that resistor, so the voltage at the ROMs shouldnt be a major concern when finding a reset reason.

If you have good +5v on the input pin, but it still resets, there is something going on. Failing filter cap could not hold the charge during a current spike (flippers). Resistance on the connector plugs. Etc..(see Chris's link). Scope would let you watch the power when the flippers are going off.

Glad you fixed it with the Kahr board, but the tech inside me think that is a bandaid and your power supply still has problems. But if it works, it works! Hard to argue to mess with the board when you have it working.

While the 4.79 might be enough to keep the watchdog circuit happy. When the flippers go off you are depending on a capacitor to act as storage tank to keep the voltage up. If you deplete the capacitor in the time of a flipper pulse the voltage dips and the watchdog triggers a reset. This happens so quick, a DMM may not see the voltage drop.

Poor physical connections add resistance too.