Try running the auto align function on your monitor first, I think that is more likely to solve the problem.
I'd tried that. It's more like the incorrect refresh rate was being used.
I've got a new PSU plugged in, and so far it seems to have fixed the issue.
Very interesting, but glad I made the topic as I didn't make the connection between dirty power and analogue display.
I'll mark it as solved for now, thanks for the fresh eyes!
EDIT: This SORT OF solved my issue. The short version is, my PSU was fine, it was the power bar I had it plugged into that was bad.
When I tried a new PSU, I plugged it into a different power bar to test; and it seemed fine. So when I redid the bench with the new PSU, I was confused to see that the issue had returned. Then it clicked; two different power bars.
It's now several hours later, I've gotten no work done, and the entire bench has been ripped apart and put back together with all new power bars, network switches, and a few new USB hubs.
I'm sure they're going to love seeing 3 hours of overtime and $200 in write offs tomorrow, but at least my monitor doesn't flicker anymore haha.