Sounds like U14 on the mpu is bad.

Swap out the PIA at U10 first. I think it controls the lamps.

I've had this problem on a few mpu's. U14 gets cooked by the large resistor that is right next to it. Lights flicker, not always causing resets though. You mentioned corrosion damage which may have more to do with the resets.

I would change out the PIA at U10 first, before getting out the soldering iron.

If you have a meter that can check frequency, pin 4 of U14 should be around 120Hz. If that looks ok then check frequency at U12 pin 3. It should be around 320Hz. If either of these are out of range, it can cause resets and flickering displays or lamps. I'm betting its U14 that's the issue.

PIA is probably ok. Check the frequencies as mentioned.

Check for 5volts on U14. Not sure which pin, check schematic. Pin 1 or 14

It is pin 1.

Check frequency at U10 pin 18. Should be 120Hz.

Zero crossing detector not functioning.
Check for 22 volts at tp3 on the mpu. If Ok, check for just under 5 volts on U14 pin 14

You Should look at the mpu schematic. The zero crossing circuit is clearly detailed. If you have 22v at tp3 then something is wrong further downstream in the circuit all tied to U14.

Maybe you should measure the voltage on U19 pin 8(should be around 3V). My logic probe never shows a pulse on R136 left leg/U19-8 on working mpu's. More like high and low both leds lit