Rule of thumb, never rule out the obvious, intermittent instability can also be caused by the PSU. A good known PSU or good known GPU with similar power draw. Would rule it out as a suspect.
bad vram, can always be tested for with tester. GPU z build in tester is another good way to quickly test stability.
It is a 3+ year old PSU, superflower is a good a brand as any, but the reality is they do degrade over time.
If you don't have a spare GPU or spare PSU, the only other way to test the stability is to underclock the GPU with something like MSI afterburner, 200mhz is a good starting point if there is still instability after reducing by 500mhz, you can reasonably assume the GPU is at fault.