If you didnt had this problem before and everything was working fine with same specs same hardware; than its either your video card or your CPU heating up. Taking the side off and blowing fans on cpu/comp wont help.
If I was you, I'd remove fan from CPU, clean thermal compound off "liquid like solution between fan and cpu". Buy new tube of thermal compound, you can get it from any comp electronics for 5/7$. You can find tons of videos on youtube on "how to apply thermal compound" Apply the new thermal compound and put everything back together. Run comp, run game, if the problem doesn't go away than I'm afraid it's your video card. It's a cheap test, you can try it before you end up with hundreds from pocket.
How old is your computer?.