Last night I was playing when all of a sudden GI string 3 (slingshots, and upper left backbox) went off. I noticed that for the last few days it would sometimes get very dim for a moment
If I unplug J121, playfield string 3 doesn't work; if I unplug J120, backbox string 3 works but it's much more dim than the other lights.

So I checked continuity for that string on the board and all seems to be good (J121-3 goes to J120-3, goes to middle Q14, goes to J115-11; J121-9 goes to J120-9, goes to F108, goes to J115-6). Voltage on the male connector looks good, too: 6,7V between J120-3 and J120-9, and between J121-3 and J121-9.

Then I checked continuity on the female connector, and there's continuity between J120-3 and J120-9 (yellow/white yellow)... but also between J120-2 and J120-8 (orange/white orange)

Do you guys have any idea what the problem could be? It's actually the first time I have to work on GI so I'm kinda out of idea.
What's puzzling me is both the backbox intensity problem when the playfield connector isn't plugged, and the continuity between the two ends of string 2

Even though the voltage on the male pins is good?

Okay, so I took out the board, added solder to J120-3, J120-9, J121-3 and J121-9. Turned it back on, and it worked perfectly for one minute, then it went off again
Wiggling the connector didn't do anything.

Both an update and a bump: I didn't touch the machine for more than two weeks. Turned it back on to show the problem to a friend, and it worked like nothing happened
Turned it again the next day, and all was good, but then I said to myself: "there's not auto-repair on this thing, let's try to check it out", so I decided to fail the Meltdown mission to test the "blackout" GI effect.
It blacked out, indeed... and didn't light back!

Should I double check the voltage on the pins or something?

It returned, string 3 doesn't turn on at all for now.

Edit: measured the voltage again, it reads 2,6 V; I suppose that's the problem, but what can I do to fix this?

Updating, since I've more time to work on it with the lockdown.

Voltage is good for the string 3 on both J120 and J121 when there's no connector plugged on both.
When I plug either the playfield or the backbox in J120 or J121 (I've tried all 4 combinations, it's always the same), there's very little power in string 3 and the voltage in the unused male connector drops to somewhere between 2,6 and 3,1V.

Since this happens with both connectors, I'd guess it's more of a board problem, but I'm really puzzled here.

Any idea?

Allright, I somehow fixed this, I think!
I replaced Q13, Q14, U1, R13, R14 and R15. Basically everything that's dealing with string 3 GI. Nothing worked.

Turns out it the issue was on the square connector between J115 and the electrical transformer. I just unplugged the thing, tugged and pulled the yellow-white wire, and said "Judge Dredd" 3 times before plugging it back.

All the GI is now working perfectly. I've been stuck on this for 2 months, so it feels like a huge let-down.