FPS Lag the more I play a game

[Doctor FIlthy]

So, The past few months, my computer has been amazing. 60-50 FPS on CSGO, 60 FPS on League of legends. But now, when I play, the more time I play, for example a maximum of 2 minutes, the fps drops drastically to 20-30 FPS. What do you think is the cause of this issue, and how do you think I could resolve it? Thank you for atleast reading this :3

NVIDIA Geforce GT 620 2GB
I3-4130 @ 3.40GHZ
4 GB ram

 

[DigitolMC]

If it's getting too hot it might be underclocking itself. Try cleaning the system out for dust etc.

 

[Doctor FIlthy]

If it's getting too hot it might be underclocking itself. Try cleaning the system out for dust etc.

Okay then, might as well clean this thing, thanks tho, really appreciate it.