Mine also powers itself down. Sometimes while in the holster and sometimes while just sitting on the counter. It does it about once a day. I have the software update. I contacted HTC and they said do a soft reset (battery pull) and if that does not fix it, do a hard reset. I did both and now in addition to the power downs, it resets itself often.
I called Verizon twice Friday 5/27. The first time, they had me do a hard reset by holding volume down while powering up. This tech denied that this was a known issue. He told me to try this and if it didn?t work, call back. Ten minutes later, it reset itself while just sitting in my desk. I called Verizon back and the second tech said that this was a known issue and that he participes in daily teleconferences with HTC trying to figure this out. He is sending me another reconditioned phone. I plan to try it for 48 hours or so before I send mine back. If it has dead pixels or won?t hold 3G like my original T?bolt, I?ll not keep it. They do not know what is causing this so there is no immediate software update in the pipeline. All they can do for now is to replace the phones. He told me that if the phone arrives with the old software to not update it.