Not done a factory reset as my phone is all nicely set up and don't want to have to do it all again. If it transpires this is software I'll be willing to do it though I agree I don't think they know what is going on. Either way it isn't tooooo much of an issue for me at present.
Have done a factory reset, and maybe 30 mins later had a reboot. I did reinstall all my frequently used apps. I am not sure if it might be one of the htc apps, gonna uninstall those and see.
I also want to know very soon what kind of issue this is so that I can just return it for a new phone instead of having to send it for repair later.
EDIT: we do know at least that it isn't some kind of flashing error as a reset doesn't help. Also, if it is an app it is probably a HTC one, since our app overlap is likely small and we know it isn't Facebook. As well as all this, it could be a firmware issue. Or, it could be hardware, but I have never heard of this kind of problem before - usually it would be related to the phone not heatsyncing effectively. It seems too common to be hardware, hardly any have sold and some people have both theirs and their partner's doing it. My money is on an HTC enhancement or firmware.