I emailed support yesterday, but thought I'd start a topic in case anybody else has seen something similar.
Basically, after some amount of time (today I got about an hour) the headset starts to lose tracking. Initially only when you look down, then when your not looking towards one of the base stations, then pretty much all the time unless you close to and looking directly at one of the base stations (presumably not long after that it'll give up altogether).
From the first flickers to being completely unusable is about 5 minutes.
Once the headset has been left off for a little while the tracking works fine again.
To reduce variables, the camera has been turned off, bluetooth communication with the base stations disabled, tried different USB ports. Mirrored views on the PC still work, with the headset orientation still working, and the controllers still seem to track perfectly fine so it doesn't seem to be a problem with anything but the headset itself.
There is a youtube video of somebody with what seems to be the same problem: https://www.youtube.com/watch?v=UqEK_PeMTeM
Where the side sensors stop working until the headset has been cooled (though I'm not sure I can recommend putting toasty electronics in front of any icy air con).
I've only had the Vive a few weeks. The first weekend I had it, it was going fine pretty much all day (with lots of people having a go), but recently it doesn't seem to be lasting before it conks out. Yesterday I had about 20 minutes before it started to go. I recall watching some 3D video with it when I first got it (The Day of the Doctor, if you're interested) and it started to flicker the grey screen a little, but I only had one base station sat on my desk so I didn't think anything of it and never sat through the whole thing.
It's hard to tell how hot the thing is getting inside, or how hot normal operation is. The faceplate doesn't get hot (except for from my own face) and the front of the Vive is warm, which seems normal. I did check that the cables where in properly and upon removing the HDMI cable the tip of that was pretty hot (I reckon ~70 degrees, but it would be very difficult to tell).
It might not even be an overheating problem as such. Maybe a lose connection on a ribbon cable as the plastic expands slightly or something, or perhaps the electrons are getting tired and taking a break.