SteamVR random performance drops?

Currently i’m working on a VR project in UE4.11 Though as we started to optimize the scene we noticed that even though the “optimised” level ran buttery smooth at a solid 90fps, occasionally the game seemed to have random moments of bad performance for about a second every 10 or 15 seconds.
I have managed to “capture” a performance drop using the profiler, though i cant seem to figure out what it is exactly what causes the performance drop from this data.

Here is the ue4stats file if it is of any use.