vRAM, sysRAM would produce very different, and far more annoying errors.

And I don't think there is any test you can run to verify what the memory is doing, but "artifacts" on screen almost always indicate an overheating problem.

The checkerboard one you have is definitly a memory error, and since your system isn't locking up hard when it happens (which it should), I'd say it's permanent damage....sucks ;[.