Games drop frames sometimes and I don't know why

I play games on my MacBook every now and then. Most of the time they work perfectly well, very smooth but for some random reason which I can't explain the games which work perfectly suddenly just drop frames to the point where its unplayable! I don't understand why its doing it because like I say most of the time it works fine but like once a week it just sucks. 

I have tried restarting my MacBook a couple times and closing all programs and tasks but it still lags like crazy! 


Try going into the game settings and looking at the fps options. Other than that, I'm not an apple fan so I can't help hardware wise.

Maybe it gets too hot?