When your gpu's usage is low, meaning when you aren't loading your gpu with larger loads, things get choppy. If I'm getting that right.
(Be sure to back up your files before doing anything!)
First thing you should check is task manager. Sort by CPU usage and then GPU usage. If you see anything overly high, keep note of it. Before you end the task, try to identify what it is. Look it up and see what it does. Once its identified, try hitting end task and then check your performance.
Your gpu might not be getting as much power as before, or possibly the vram speed is defaulting lower than intended. Or something isnt running as it should. I'd doubt it but you can never confirm until you know. I would
-Check your parts to be sure they are running the right speeds.
-Check bios for ram speeds set to their normal and same with your cpu.
-Reset bios to defaults and check to be sure your XMP is set.
-If you clean your pc often, be sure you check all your cables are properly in place and pushed in.
For your GPU, if you have MSI Afterburner or similar program, try increasing your gpu's power limit.
If you think the outlet is the problem, try another.
Also don't forget to run a virus/malware scan. I use Avast and Malwarebytes personally. If you haven't done so I would recommend.
And if you're sure it isn't a hardware you plugged in, it's always check any usb extenders, splitters, or anything that can draw a lot of power. ( purchased a USB hub the other day and it does similar things to my pc and ends up crashing it. Can never be too careful.)
Check these off your list and be sure to revert any changes you might make that don't seem to fix the issue.