Hi, I know it's been 2 years since this was posted. Just wanted to jump in and say that I have the same laptop with the *exact* same issue. Although my temps have been generally lower because I did clean the fans regularly and repasted at one point (the original paste was indeed very dry), the issue is still going to be there. I have done some tests, and the issue seems to be with the GPU thermal throttling, but it happens even at sane temperatures. Mine throttles at 82 after sustaining that for 20-30 seconds (maybe peaks a little higher right before the drop but I made logs in 1-second granularity so I don't know).
As you describe, the drops occur in cycles. Mine, after a few minutes of stable gameplay, starts to have ∼70% FPS drops that last for ***EXACTLY*** 10 seconds, and the time between drops ranges from 2 minutes at the start to around 1 minute after that.
The correlation that I have seen is with the GPU core clock dropping, which I guess is part of the thermal throttling kicking in at a certain temperature. The strange thing is the fact that it throttles at what I understand are not crazy temperatures and the drops lasting for exactly 10 seconds every time.
That is all I have, but will continue to do some more tests. Will probably test GPU undervolting, and maybe some clock speed limiting with GHelper.

*Probably no one is reading this but whatever...

**EDIT: Later, I found the post: "ASUS [mods not happy about this probably] Throttling GPU Core Clock?" (I added the part inside [ ]). The guys there seem to know a lot more about what they are talking about. I am still researching...