Unreasonable double FPS drop. Is that GPU? How it works? PollGameDeviceState.

Hey All!

Working with Oculus 2, Unreal 2.46. CPU - i7-5960X, GPU - GTX 980Ti
Using deferred shading.
Scene is pretty simple - BSP standard floor, VR camera+controllers, stationary poin tlight. Sometimes + some geometry sometimes just that.
When I start the game FPS 90 - 11 ms. It can be faster, but, as I understand Oculus driver or Unreal Engine limit it to 11 ms.

When I look at the floor - FPS become 45 - 22ms. It looks like switch, not like smooth load. Sometimes it happens without any actions from my side.
I can be like a waves, 90 then 45 and then 90.
In task manager I see the CPU is not loaded, and GPU drop down load from 90-98% to 50%. Here is two states:

and 2)

I made a profile and found that the most f the time takes process PollGameDeviceState.
Here is the screens of profiling, game thread and Render thread:

So the question is - it is my GPU cant render the picture on 90 FPS and UE switch FPS to 45? How it works?
Or some device takes more time and other systems wait for an answer for request?
Also I saw the game via Intel Graphics Analizer, but that didn’t resolve my questions Could anyone give an advise what else to check? Or explain how that FPS drop works? Why on that very simple scene it happen? 980Ti is not so good? Or the reason is different?