Well I thought my problems were (more or less) over, but apparently not. I have now decided the Vive Cosmos Elite is the single biggest piece of tech crap I have owned in the 25+ years I've had computers, and at NZ$1800 a ridiculous, expensive fiasco. I am fully expecting to return it for a full refund.
The last two days I successfully got the headset started after 3-4 attempts (5-10 minutes), unplugging and replugging the headset power cable several times. I then was able to play Half-life Alyx for an hour or so each time.
Today, I cannot get the headset to start at all. I have spent over an hour unplugging and replugging it to no avail, restarting Steam, restarting my PC etc etc. The headset, controllers and base stations all have the latest firmware. About half the time I start up Steam VR, the Vive Console also starts, and then says "updating headseat". It then puts up a second box also saying "Updating Headset, Please do not disconnect devices or force quit the Vive Console". This even though Steam VR shows no updates available. After 3-4 minutes, the Vive console says the headset is disconnected and gives a 001 not connected error in red.
The other 50% of the time Vive Console doesn't start at all, just Steam VR, which says everything is connected OK, and the headset is currently tracking. Sure enough, if I open the VR View window thru the Steam VR menu, it shows on my PC desktop what I should be seeing inside the headset - but the headset itself remains black, with the red light on. If I stop moving the headset around, it goes into standby, but if I then pick it up and move it, the VR View window again shows the correct view.
This is so frigging frustrating.
A clean Windows install isn't really an option, as I have lots of customised work software, plus over 800gb of games which with my monthly data cap would take over a year to download again. And anyway, it shouldn't be necessary. I know the damn thing works, it's obviously a connection problem. HTC needs to own up to the poor quality of their gear.