My wife's EVO 3D started randomly rebooting. This was her second 3D too. The first one, the screen went a weird rainbow snow, and the phone became a brick. So when her second phone started with the random reboots, she got seriously annoyed.
I decided to call HTC and see what advice they could offer. A tech walked me through a factory reset, which we were kind of loathe to do as it would reset all my wife's customizations. But we did it, and so far so good, it's been a couple days and she hasn't noticed any reboots. It involves powering the phone off, then holding down the down volume key while powering it on. That gets you to the boot loader, where there's a factory reset option.
I wonder if the cause may have been the particular sequence of updates that HTC released, applied over time. Her phone definitely had HTC updates appear after the reset, and we applied them all in one go. It's been stable as far as I know.