Have you tried running a software repair via the Update Service or PC companion. As this will basically reset your phone to factory settings, you need to make sure you have your data saved, eg contacts saved to sd card or synced (or both if you are like me and want to make sure!) Any apps can be reinstalled from the market.
This often irons out any glitches. If it doesn't sort it out you may need to have the phone looked at either by your carrier/retailer or SE.
