4.7V was way too low, I am surprized it booted at all.
Are you measuring the +5V voltage on the power supply, or on the MPU across the electrolytic cap near the power connector
(this is where I typically measure the +5V to see if the power harness/connector has good continuity without excessive loss).
For the board not booting reliably at +5.1V, or not immediately after a power down.
Check AC ripple on the power supply +5V out, voltage arriving on the MPU, then check the power reset circuit on the MPU.
For the dead displays, with the power off, I would try reseating a few times the large J3 connector on the MPU's right side,
and see if there could be corrosion in that connector causing an issue for the digit drivers output at the MPU (and the +5V/GND) feeding all displays.
If not that, perhaps a digit driver chip went on the MPU - hopefully not.
Looking for Bally Star-Jet, Capersville or The Wiggler, Williams Magic City, Grand Lizard, Swords of Fury, 1986 Pinstar Gamatron, and a few others.
Collecting & repairing electro-mechanical and solid state pinball machines with quality and care for over 35 years.
Serious and fun hobby, powerful nostalgia !