Dell XPS 15 9560 FPS drop during games

I recently purchased the Dell XPS 15 laptop with the intel i5 cpu and Nvidia Geforce 1050 gpu. When playing games like overwatch on high settings the fps will start off at 90+ fps fairly consistently, but after playing for a while the max fps gradually decreases and soon ends up at around  50 or less. I have searched and seen people suggesting thermal throttling, but I have used MSI afterburner to check and the gpu clock seems fairly consistent and the temp is around 77 degrees, although I am not quite sure as to whether the this is the problem or not. Any ideas on how to resolve this issue?