FPS drop every ~10 seconds, graphics card?

Question

Cooling systems

Hello.
Faced such a problem.
The computer is not the newest. When buying, everything worked without problems, but after a while I noticed that fps began to sag in games about every 10 seconds. This is especially noticeable when you get to places that require a lot of resources from the computer (for example: a large number of players in the location ((moorpg)).
This computer was rarely used, so this issue did not arise before, although the problem existed.
On the computer due to another problem, the hard drive was replaced, so I don't think it's the problem.
Video card gtx550ti. When playing, for example, DotA 2, it heated up to 81-85 degrees. When testing with full mark at 720p, the temperature at half of the test was ~87. Then I interrupted the test. I was afraid for the video card.
When the service changed the hard drive, they added a cooler at the same time. They said that one was missing.
When there is no load on the computer, the temperature of the video card is normal.
What could it be? Maybe the thermal paste is covered or something? Or the video card itself?
I don’t know much about computers, so I didn’t climb in advance and check everything.