Mine was doing this and annoying the heck out of me. If you want a temporary fix to it (until HTC fixes it with an update or something), try this:
1. Go to the Market and download Keepscreen.
2. Once it is installed, open it and click on "Applications"
3. Select Dialer, Dialer Storage, and DM Command service.
4. These selections will turn green letting you know that you are good to go.
That's it. It works very well for me.