Power measurement, that depends on where you measured. Should be seeing close to this:
MPU IJ17 Pins 4,5,6 = +5.03 VDC @8.1 mV AC ripple
U26/U27 ROM Pin 1 = +4.91 VDC
If you measured 4.86 at the PSU you are going to probably find even lower at these points. BUT, the common symptom of low power is random resets and lockups. The failure you are seeing, my guess, is corrosion related most likely. If not, it could be another IC that is on the address/data bus that the RAM is having trouble with.