Happened for months with my girlfriends phone. Had SIM card issues, random locking, random "spooling" (where the little dots just continuously go across the top of the screen) and random reboots or just shutting down. She missed many a phone alarm because it would just shut off during the night (while plugged in!). I finally took it to the local AT&T warranty store and they ran it through tests. Turns out the phone had a bad radio in it as it wasn't getting signal (the tower is right behind the building and it was getting 2 bars). It also had a myriad of issues (scrolled horribly, locked up, etc) while the guy was looking at it. After about 10 minutes he replaced the phone with a refurb. Now everything is peachy and her phone works darn near perfectly. It's still an HTC so I would never use it but she likes the form factor and the color. Yes, I acknowledge I was a bad boyfriend and took months to get it fixed. I feel shame.