You may have a corrupted entry in the database. Have you done a backup of the device, executed a hard reset and tried it with nothing on the device? If it does not reset, then the contacts data base may have a problem. An application that works with this situation is DBFixit from
http://pimlicosoftware.com/dbfixit.htm. In the test drive mode (non-registered) it will tell you if there is a problem. In the paid for mode (registered), it will probably fix it.
You also have the option of doing it from the desktop by over writing the file on the device. Whatever way you do it, make sure you FIRST back up the device.
Ben