Post some clear high resolution pictures front and back (preferably taken outdoors in shaded daylight) of the MPU board.

Are the flickering controlled lamps LEDs or incandescents?

You'll need a logic probe to diagnose this.

Your problem sounds familiar:
https://pinside.com/pinball/forum/topic/big-game-mpu-200-issues#post-5616331

I can see some reflowing of pins on the rear side except for J4 and it could be that i can't zoom in any more but at pins 1,2,3 & 4 look to have cracks around them.....but i could be wrong.

maybe reflow these?

Ok, so you're not dealing with battery corrosion.

How about a full frontal picture of the board?

Per the thread I linked above with the same symptom as yours, what does your logic probe indicate at pins 3, 4, 5 and 6 of U15?

Without checking a board here, your readings look reasonable.
Pin 4 of U15 should be pulsing which happens upstream via pin 10 of U19.

The output of U15 at pin 3 is called the "VUA-Ø2" signal. It's used for timing when devices on the MPU board are accessed. Check the state of it dowstream at each destination:

U10 pin 25
U11 pin 25
U7 pin 10
U7 pin 13
U17 pin 10
Non-banded side of diode CR52

They should all indicate the same as U15 pin 3.

It could be the light from the pic but zooming in J4 pins 1 & 2 appear to have a crack around them, and I get that you are happy with the appearance and I don't know the workings with such detail as Quench but seeing the board is out of the pin you got nothing to lose by reflowing J4.

Any chance you have an oscilloscope? Might need to get a good look at that signals waveform.

BTW, your logic probe readings indicate your logic probe might be set on CMOS. If so, please change it to TTL.

In the mean time, can you probe the following and let us know what you get:
U17 pin 10
U17 pin 9
U17 pin 8
U8 pin 19
U13 pin 19
U14 pin 1 through to pin 7 inclusive

And have you got an EPROM programmer? Would love to get a copy of that V2.0 U2 ROM..

Sorry to be a pain, can you redo the U14 readings with your probe set to CMOS? It's a CMOS chip and some of those U14 results look a little off.

I would expect in the order of flow:
U14 pins 6 & 7 = high with pulse
U14 pins 5 & 4 = low with pulse
U14 pins 3 & 2 = high with pulse
U14 pin 1 = low with pulse

Your U17, U8 and U13 results look reasonable.

Can you dump that U2 EPROM and tell me what checksum you get in your EPROM programmer software?

So those results are pretty much what's expected. No clues have been revealed yet.
With regard to the lamps flickering, does it happen all the time or does it come and go? Does the game intermittently reset within a few minutes if you leave it in attract mode?
Is there any background you are aware of with this MPU board? i.e. did you get it this way or did it it start failing in your possession, and what work has been done on it? I can see U10/U11 sockets are new, U7 U8 and U13 are replaced.

The widely available U2 ROM has a checksum of 000208DE. You have something different and the Stern documents (in German attached) indicate there are two Galaxy ROM versions. They indicate your U6 ROM will be different in conjunction with your new U2 version.
It would be great if you could dump the four ROMs so we can have this new version available for use/preservation.
Thanks.
Stern_ROM_list.pdf

Do you by chance have something (frequency counter) to measure the frequencies at the Zero Crossing Detector and Display Interrupt Generator?
Pin 18 of U10 and Pin 40 of U11 respectively. Should read 120Hz and 400Hz.

If your programmer supports 2532 EPROMs can you try reading it as a 2532? The file will be double size but hopefully one half will contain the data.
If it doesn't produce valid data then yeah a converter needs to be made to either swap pins 21 and 18 to make it 2716 readable, or connect pin 21 to pin24 (5 volts) to ensure it will read proper as a 2532.

If you go into lamp test mode, do the lamps flicker when they flash on?
Are you getting wrong lamps coming on during attract mode / game play?

With your logic probe, what does it indicate at pin 1 on any of the 24 pin chips on the lamp driver board in attract mode? From memory it should be Low with Pulse.

Let us know when you get the oscilloscope ready and post a picture of it so get some idea of what you have.

Looking at your attract mode video, there are abnormalities with respect to the top saucer lamps, drop target lamps and bonus lamps chase sequence - sometimes they stall/speed up.
Compare to here to TNT's video where those lamp sequence is smooth:

This is yours:

if you have isolated the problem to the MPU board I would try a known good PIA at u10 as it controls both switches and lamps. The power on self test is not fool proof and a bad PIA can pass the test.

Great to hear you found it!
Can you remember which U14 pins were off? May give us a better understanding of the issue.

Thank you so much for posting your fix and following through with tests. This will probably help more people in the future!

Thanks, if you still have the machine, any chance you can logic probe these pins on the new chip?

U14 pins 6 and 7
U14 pins 4 and 5

Your original results were slightly suspicious so I'd be very curious if they indicate differently now.

This is an effective approach, I have done it in the past mostly with the 4543 decoder chips on the display boards. It's a way to determine if a missing display segment is caused by a bad output on the chip (which can be easily compared to other outputs on the same chip).

Cheers, so pins 4 and 5 have changed from having a very fast pulse to just a pulse.

U14 pin 7 is connected to pin 6 so should have had the same reading, but we'll just put that down to a bad connection on your logic probe when taking the reading.

Thanks.