FPS varying massively / Present Time in D3D11RHI spiking /

Hope you can help me to track down a strange performance issue. I had real difficulty obtaining a stable frame rate in my game, so I have now just created a new blank project with starter content (UE 4.6.1). Launching the blank project standalone gives me 200+ fps but i notice the fps constantly changing (no movement of camera, just looking at the two chairs).

I ran stat D3D11RHI and it shows everything reasonably stable except Present, which has Min 0.2ms, Avg 0.9ms, and Max 13ms. I then ran stat startfile to get a profile which does indeed show Present spiking.

It is my first encounter with Profiler, do you have any idea what the problem is that’s causing the spiking, or any tips on how to track it down?

I’m running Win7 Pro x64, i7 975 @ 3.34GHz, 12 GB, 780 Ti. I have two monitors and DK2 connected. the above problem occurs regardless of second monitor disabled and Oculus service stopped. The app is running windowed so not using DK2.