A grey screen typically indicates a tracking issue. I would recommend trying the following:
Switch between USB ports to see if the USB port bandwidth makes any difference.
Ensure your basestations are mounted according to our guidelines. If there is a dead-space where tracking data cannot be recorded, this can easily result in a grey screen as you described.
Check SteamVR's UI to see if an error message is displayed when the grey screen is displayed
Check your frame timing during the outage to see if your GPU is being over powered. If you go to SteamVR -> Settings, in the bottom lefthand corner you'll notice a graph with a time indicator. If you're above 11.1ms your GPU is struggling to render that content at that resolution at 90FPS. If it starts getting above 20-30, your GPU is extreemly underpowered and may revert to a grey screen to prevent motion sickeness.
Check for reflective surfaces and cover them up
Check SteamVR's system report for Reflective surfaces. Generate a SteamVR system report via SteamVR -> Create System Report.
Next, save the report to a text file; open the text file in an editor, and search via Ctrl-F for the following term: back-facing.
Here's what a reflection looks like within SteamVR's logs. If you find this, you have a reflective issue.
Sun Jun 26 2016 23:02:09.676 - lighthouse: LHR-4E5QF209 H: Dropped 32312 back-facing hits, 2069 non-clustered hits during the previous tracking session
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