This may be the problem that I was having a few months back. My Surround would randomly freeze and reboot every once in a while, but over time it got worse and worse. Eventually it would freeze and reboot itself every 2 minutes or less (it was truly maddening). The only thing that would fix it would be to take the battery out and let the phone sit for a while. Of course, that was only a temporary fix.
Put in a technical support ticket for you phone on www.htc.com. They'll respond via email and, based on your symptoms, tell you to call them. Their number is:
866-449-8358 (North America)
6 a.m. to 1 a.m. Eastern Time (every day)
Fortunately my phone was still under warranty, so I only had to pay to ship it there (their repair center is in Houston, TX). But they fixed it quickly (and you can track its status on their website) and got it back to my doorstep in less than 10 days total IIRC. It was a really smooth repair experience.