Once, years ago, when I was still working, we had a problem with an
HP/UX workstation that flickered badly.

Finally, we replaced the entire machine, and the flicker was still there.

So we replaced the monitor. The flicker was still there!

To make a long story short, it turned out that when the user had moved
the desk in her office, she had unknowingly put the old CRT monitor
(all we had back then) near a large AC power cable in the wall behind
the monitor, and the magnetic field from the cable was affecting the
monitor!

Unlikely to be your problem, even if you are still using an old CRT
monitor, but just a note to check for totally unexpected things...