Question Why FPS is falling & game is getting stucked

Apr 3, 2019
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About a week, I'm facing this problem. Whenever I start to play game on my PC, It runs smoothly & it shows 100+ FPS (splinter cell blacklist). But after 1or 2 minutes later the FPS starts to decrease & it comes to 2-4 FPS & the game stucks then crashes. What's the wrong with my PC? GPU temperature doesn't cross 60 while I'm gaming, CPU shows maximum 50 degrees with full load.

My configuration is,
CPU: core i7 4790k
GPU: GTX 980ti
RAM: 16gb
SSD: Apacer 120gb
HDD: Hitachi 500gb
PSU: Gigabyte 600W
Cooler Master hyper 212 as CPU cooler.