If you are using a Vive, try to disable FluidSync. Among other things it caps the frame rate at 45fps and one “timewarped” frame is inserted between to real frames. Unless you can reach stable 90fps in a game I would suggest to leave it on though, stable 45fps with an added artificial frame often appear more fluid than fluctuating 50-80fps.
With Oculus Rift a similar thing may happen with recent versions of the Oculus software if it decides that the GPU workload is too high. Unfortunately Oculus decided that they know best what to do, so that can’t be disabled anymore. All you can do is reduce the GPU workload until the Oculus software decides to allow more than 45fps.
Not sure about your 10fps dips though, sorry. That’s not related to any of the above.