Find version of mats mods for that era.
Run.

Most likely memory is faulty somehow.

It's either a VRAM chip gone bad (one of chips has "stuck" bit), a BGA connection issue between VRAM and GPU (can be under either GPU itself or VRAM chip) OR NVIO chip giving up the ghost (with again bad connection of failed transisotors inside).

Note : NVIO chip is located to the left of GPU core on G80/GT200 cards (top view, with PCIe at the bottom), and it's in short responsible for displaying things on monitor.

myne and agent_x007 - thank you very much for responding, really appreciate it.

I am having trouble locating nVidia MODS/MATS that would work with this particular video card model, but I will keep searching.

From what I understand (this whole situation is well above my pay grade, so to speak) the only chance for repairing this card is to replace eventual bad VRAM chips, right? Meaning I have to find GDDR3 512Mb chips and also a technician who can do the replacement? Not gonna be ease, honestly...

Also, is it correct to assume that the NVIO chip that agent_x007 mentioned is not repairable?

Thanks again for the responses 😀

If you're REALLY lucky, that's a VRAM problem and you can replace it relatively easy. But usually G80 chip itself have some sort of bumpgate failure (you can read about it here on Vogons if you search) and there's nothing you can do about it as a long term fix, unless you can find a new old stock of G80 chips.

At the same time, it's usually just not economically reasonable to fix any of these cards when you can just buy a G92 card really cheap.

It could be many things.
Mats mods usually covers a few generations backwards.
You might have more luck looking for 2/300 series versions.

Thanks again for all the responses, it's been an eye opener 😀