I noticed in 1-2 games the FE was doing something weird - in the odd spot the core clock speeds would suddenly behave differently to any other time followed a moment later by the framerate dropping low for a moment even if the clock recovered - if the processor has any kind of power management enabled rather than maximum performance set it will also clock down under the lower load if that is corresponding to less pressure on the CPU when that happens. At a guess there is some kind of hiccup with the HBM2 and/or shader caching that is stalling something momentarily at GPU driver level - I'd guess it will be ironed out come retail release of the RX Vega.
The Fury does the same thing, it's power management isn't very good, it kills performance, that's why that dude did that app called "clock block" because the clock was bouncing around everywhere then AMD put the power saving on/off option in the drivers, and to this day it still has to be on the off position for gaming otherwise it affects performance (well at least last time I tried to use the power saving feature). Then they came up with chill which I haven't tried yet but apparently works ok, but only on a per game basis.