HELP!!! Dramatically sags FPS in all games

I bought a new system unit, install games, start playing for a minute or two and the fps drops from 60 to 9, then rises again and falls again and so on in all games .... what to do xs
Operating system Win 7 (reinstallation did not solve the problem)
Video card radeon r9 280x 3072mb (new firewood) (https://market.yandex.ru/product/10632170?hid=91031&clid=502)
12 gb RAM
amd fx 4100 quad core processor 3.60 ghz
600 watt power supply
please help, otherwise even Don't play War Thunder =((