AMD R9M370x with GTA5

So, after using it a bit I can see that the game works perfectly fine for first 10-15 minutes on considerably high (not ultra) graphics. After that it starts showing very low fps at times and this happens repeatedly after each 40-50 sec.

I am really ****** off. Is there any solution to this problem?

FYI, I have a MBP 15, mid 2015 with 2.5 Ghz Intel Processor, 16GB Ram, and 2GB AMD R9m370x and Windows 8.1 Pro.