Slight slowdowns in VR playback (Vive) - Sorted, my mistake!

Hi guys,

I've been noticing the framerate seems to vary a little sometimes when I am playing back the scene in VR. It's only slight but sometimes it will just slowdow a little like it is under load and then speed back up.
If i look in the editor window though it seems fine...
Any ideas what might be causing this?

Thanks,
Pete