are you sure you mean GI lights (general illumination) ?

the #555 bulb is for the solenoid expander board relay.

maybe check the GI flasher board under the playfield, dry solder joints/connectors?
located under the playfield, about in the middle, a 6 way connector with 4 wires/pins attached, under the ORBS targets

are you using all incandescent bulbs or LED's, or possibly a mixture?

did you check the flasher board headers for dry joints?

the bulb for the flasher board under the playfield is driven by a transistor on the lamp driver board.

the bulb for the solenoid expander board is driven by a transistor on the solenoid expander board.

the SCR on lamp driver for the lamp flasher needs the #555 bulb in the circuit for the voltage drop needed to operate.

so in lamp test do you see the #555 bulb flash?

whilst probably not related, in solenoid test do you see the #555 bulb flash in solenoid test?

I suggest putting the #555 bulb in for the flasher board and working out where else the problem is?

just a thought but sounds like a dry solder joint/or more causing the intermittent oddities.

The SCR Q14 on your lamp driver board J1 pin 18 is responsible for driving the GI Flasher.

The MOC 3011 you changed on the GI flasher board needs the nearby #555 bulb as a voltage drop for it to work.

I suggest checking Q14 is working and looking for any dry solder joints on the rear of the headers on the lamp driver board, particularly J1 pin 18.

If the above fails then the triac mounted on the panel in the base of the cabinet may have failed.

Great news!

Well done