Not 100% sure with Gottlieb EM's but Williams wired the G1 (general illumination) via the hold relay , so the 'lights flashing' intermittently would suggest a problem with the hold relay .
On one of my Williams EM's I am currently missing the hold relay coil (cause its too burnt) so have simply bent closed a few of the switches (such as the GI) so game still works until I buy a replacement coil .
It just means that if you switch the machine off mid game and switch it back on , it stays in mid game instead of stripping the game over relay.
You would, of course, need the game manual or schematic to be able do this workaround if needed
