None of my current HDMI's have a small enough head to fit into the front of the headset unfortunately but I might just pick up a cheap one tonight for testing.Try a different HDMI cord. Don't use any extenders. See if that helps. I had the same issue.
I have sort of been thinking it's power fluctuations (tried different circuits) but the headset has had a sensitivity to being picked up as it usually switches off (and led goes red). Been very careful with how I winded up my cable and not stepping on it etc (and it was working perfectly for initial two hour session).
Anyways: I have a support ticket up with Steam w/diagnostics-file given. Will pick up small HDMI cable tonight.