Question Lagging while gaming to about a frame per second after a while, no overheat

[alphakp295]

Hi,

My system recently has given me problem while playing games. The system should not lag, but it lags while playing games. Now, I only played 3 games recently... Forza 7, Fifa 18, and Rocket League.
When I played Fifa 18, it plays fine at first. After about 5 min into the game, the game start to lag. It drops to 1 frame per second, and it doesn't even take any in-game command. But I can alt+tab and get out from the game no problem. When I also play the rocket league, it does same thing... after few minutes, it lags like crazy, and it comes back after few seconds. It does that few times, then it lags like crazy and not accepting the command like in fifa 18. Now, when I play Forza 7, it works fine... Only played about 30 min, but this crazy lag happens in other games about 5 minutes of so into the game... all games are installed in same hard drive.
When I check temperature from HWmonitor, the CPU stays around 65. GPU stays around same.
What would be possible issue for this crazy lag??? hardware issue or software issue?
Spec: i5-8600k, 32gb ram, gtx 1080ti.

edit: just installed and started hitman 2... it lags like crazy from start... then alt+tab and goes back to game make it normal again right away... then lags again after few minutes... anyone got an idea??

 

[rgd1101]

cpu/gpu temp and usage during the game?
ram/ssd/hdd usage during the game?
malware scan the system?

 

[alphakp295]

Cpu temp 55-75. Gpu temp 60-75. Usage both from 40-100.
Ram usage 8-18gb. Ssd/hdd usage 0-20%.
malware scan using windows default one and avast shows clean.