Just eliminate possibilities methodically and you'll get your answer. I've certainly never seen it before, but I've seen all kinds of weirdness that turned out to be environmental.
Once your GPU renders a digital image, it gets converted by the DAC on the card. Then it's sent to one of two DSUB VGA-ports, (either direct or via a DVI-to-VGA converter), and finally into the monitors where it hits another DAC.
So, you could have:
- a faulty GPU DAC
- a faulty converter (if you're using one)
- faulty cables
- faulty monitor
To eliminate each issue one at a time, find an identical workstation with a willing volunteer (I assume the company you work for buys x identical boxes and farms them out to people) who can assist. You don't need admin rights for any of this testing.
- start by swapping the converters, if you're using any - since these will be the quickest and easiest to change. If the black marks appear on his after a week, it's a dying converter.
- Then swap the graphics cards. Whilst more work than changing cables, I would choose that as the most likely culprit.
- Then swap the cables. It's unlikely to be cables - since the chance of two identically faulty cables on the same machine is pretty low.
- Finally swap the monitors. As before, two monitors both exhibiting identical faults at the same time is unlikely.
Assuming that you eventually do all of this, You can pretty much rule out everything except the environment.
Look for sources of heat, kinks in your desk's cable routing, damaged power cables or extension leads, f'ckin' magnets (how do they work?), or other things that are different between your desk and your volunteer's.
If you can't find anything you might as well just live with it and assume your desk is haunted.
...otherwise it's PEBKAC - the error ID is 10T