For some reason reformatting the SD card while in the phone seems to fix many problems, including spontaneous reboots. Before reformatting I recommend that you power off the phone and remove and re-seat the SD card.
Reformat SD Card
Back up the contents of the SD card before formatting!
All data on the SD card will be lost and can not be recovered.
From one of the home screens:
- Menu
- Settings
- Sd card & Storage
- Unmount SD card
- Format SD card
When the SD card is mounted as a USB drive on your computer you can not see the contents with your phone. So you won't be able to change ringtones, view pics, etc. with the phone apps until you disconnect the SD card from your computer.