I'm not convinced malware has been completely ruled out of the equation. Sounds like something is eating at your GPU resources (as well as your CPU/memory), causing the poor refresh quality. Your issue could very well be hardware/driver related, but ruling out malware is absolutely critical at this point.
What happens when you're running in Safe mode (with networking)?
At this point, I'd take these steps:
- Reboot into Safe mode with networking. Take note if the problem is persistent or not while in safe mode.
- Run RKill. Be careful of false positives. (More information on RKill here.)
- I'd recommend also scanning with MBAR for rootkit checks.
- Run your traditional protection software after making certain its database is updated for additional insurance.
- Reboot and run Windows normally and if the issue persists look toward hardware/driver issues.
Additionally, I recommend enabling image pathname for task manager (View>Select Columns). This can help point out some abhorrent programs.
Best of luck!