Strange decline in PC performance.

The problem is this: not so long ago on PC in games (for example, Star Wars Battlefront, Battlefield 4, GTA V, Fallout 4) everything worked perfectly, at ultras or highs, without lags at 1080p (although the monitor is 4K). But a week before the new year, performance began to decline in all games from GTA V to Just Cause 2. When the PC was restarted, all games stopped lagging, some temporarily, and some even for the whole day. Of course it would be possible to play this way, but I decided to show the computer to specialists, tk. I did not find any information on the Internet. They looked and said that a possible problem is the BIOS is incorrectly configured. We returned everything to the default settings, tested it, GTA V flew at ultra in 1080p (although I had high settings before the problems). Of course, everything was done with a connection to other monitors, among which there was 4k. When the PC was at home I was happy to launch my favorite GTA V, and there were 3-8 FPS. It doesn't get any better when you change the settings. Even at 800x600, stable 3 fps. Then I decided that the problem is which monitor is connected, they say, it does not pull PC 4k monitors. Ok, I took a smaller monitor (to test) the situation has not changed. It is not possible to play games at this time. After rebooting, the lags are missing only for the first 5-10 minutes after starting the PC.

Here's the config of the PC:
OS: Windows 10
Processor: Intel (R) Core (TM ) i7-4770 CPU @ 3.40GHz
RAM: 32 GB
Graphics card: nvidia geforce GTX 750 Ti (2GB)

Need help because no dances with tambourines help, and "experts" cannot determine the problem, even if you call a psychic.
Thank you in advance and I apologize for talking a lot, though I'm afraid on the PC, I just collected it.