PC slowing drastically after short idle

I apologize for the lack of details here; I'm not particularly tech savvy, but I'll do my best to explain the problem.

For the past two days, whenever left idle for even a few minutes, my PC will completely slow to a crawl. It began as an inability to wake from sleep mode, but now results in any running audio and video skipping and stuttering, pages taking upwards of 2 minutes to load, and attempting to run virus scanners or attempting defrag moves so slowly that they'd take days to finish. I have restored to a point before the problem began, but no luck. A hard reboot will usually fix it until the PC goes idle again- however, using 'shutdown' or 'restart' do not clear the problem, and they persist on startup.

Doesn't seem to be the hard drive going or a heat problem, PC is not hot and aside from being slow it doesn't show any other signs of hard drive failure that I was able to look up. Anyone have any ideas? I've tried numerous fixes I've located about other people having the same and similar problems, including disabling sleep and hibernate modes, running stronger defrag software, etc - attempting to use the built in troubleshooting software always comes back with no errors found. The only things I'm noticing out of the ordinary aside from the slow speeds is that physical memory is hovering right around 45% - I don't have any more programs running than normal, and closing excess background programs didn't seem to help any.

I have no idea where the problem originated. If it is relevant, some games I have run would drop their framerate after being idle for months now, but those issues only affected the game that was running and did not bleed over to the PC. Rebooting the game always fixed it. Have not run any games for several days now, so I'm doubting that they're related. I have not downloaded any new programs or anything that may have come with a virus attached- I'm really not sure what else to try.

The computer is several years old (I'd ballpark it about 2012) but I have never had any problems with it before. At this point I'm wondering if restoring to the factory defaults is the only option available, or it anyone else has any ideas that might get this running smoothly again.

Many thanks for any and all assistance- like I said, I'm not super tech savvy so let me know if there is any more detail that I can provide.

Sorry for the slow response;

I attempted a chkdsk /r, stalled pretty quickly (definitely stalled, not just working slowly)

Followed that with a defrag on bootup to see if it was possibly my pagefile. Seems to have worked - left my system idle for several hours and no slow-down. I'll go ahead and mark this resolved; if it starts doing this again, I'll try to gather more info and test the power supply before I seek further assistance. Thank you!