Rule of thumb for this kind of troubleshooting would be to:
- Use a program such as Driver Sweeper after you uninstall the graphic drivers through control panel and then reinstall
- Return clock speeds to stock if OC'ed / downclock
- Swap out GPU
- Swap out PSU
Unfortunately, it's not usually an easy solution since we all don't have a spare GPU or PSU lying around to source out the problem. While you have found that the issue transfers itself from one PCIE slot to the next, it really doesn't narrow down the usual suspects as to what is causing the artifacts.
Since it appears to be getting worse and if no software changes were made during the period it went from bad to worse, my wild guess is it's the GPU and some of the solder joints may be getting loose(r) in which case it would be an immediate RMA unless you want to
throw it in the oven.