GTX 1070 FPS drops?

In your case, the frame drops are from excessive fps. Weather the game has unlocked fps or not, it was still built for 60fps. Don't get me wrong, anything above 60 is great, but you notice drops more as well. Especially since you have such a high OC. Your gpu is most likely doing Dynamic Throttling (not to be confused with Thermal throttling which is where core temps reach dangerous levels causing the clock to drop to base frequencies). Basically, your PowerLimit, TempLimit, and CoreClock are dynamically fluctuating to maintain the OC. Small dynamic adjustments that the gpu does sometimes equals to 5-10 fps drops at frames beyond 60fps. My advice.. Go to Nvidia Control Panel, select Manage 3D Setting, scroll down to VSYNC, and select either SMOOTH (its like vsync off, but without tearing) or ADAPTIVE (vsync only kicks in when you hit max refresh). Also, you don't need such a high OC to play this game (unless you're doing 4k 60+fps). Hell, you can UC (underclock) the gpu and still get over 60fps. I've ran it at 60+fps at 1000mhz. As long as you have atleast 4gb of vram, you're good. Hope this helps..