sudden drop in fps

I did not know and am still not completely sure where it was worth unsubscribing, to the hard or software section, but having linked the latest events of cleaning the system unit, I decided that it was still in hard, although I was not completely sure.

the essence of what.
in any game, be it vermintide, or hot, or whatever, after about 3 minutes after starting the game, no matter what is being done or happening in it, the fps drops sharply to 15-20. while the computer is cold, the CPU load is not 100%.
restarting the game does not help, only restarting the computer.


it surfaced after I performed the maintenance of the system unit - cleaning, replacing thermal paste, nothing unusual with which I would not be familiar.

one but. this came up once, but very quickly disappeared when I decided that it was allegedly overheating and climbed to take out the vidyuhu in order to clean everything and check the thermal paste. after which the problem disappeared.
now all this looms in some kind of picture, but there is no desire to guess on the coffee grounds at all.
at first there were even suspicions of the power supply - disassembled, visually no malfunctions (swollen conduits, poor soldering, soot) were found, assembled.

inspection of vidyuhi also gave nothing.
generally have no idea from what and in which direction to go, decide by software, or hard.

I hope for help

motherboard z77-d3h
percent i5 3550
vidyuha msi 7950
RAM 2 Kingston
4giga 1ssd + 2xdd
power supply fsp gold 700 watts

Win7 x64.


UPD the
problem was solved,
it turned out that it was all the same overheating of the video card. the essence was in thermal paste, alsil3, which came to replace the old mx4. As a result, the new Deshman thermal paste could not cope with the transfer of such an amount of heat, as a result of which the radiator of the video card was cold, and the back side of the board where the GPU was installed simply boiled, while this all happened in the first half a minute from the moment the system was started.
and the drop in FPS was expressed by a sharp decrease in the operating frequency of the memory and GPU on the video - a protective mechanism against overheating