I went through a several month period of time where my new BB 9650 would reboot every couple of hours. This went on for 4 months. Here is what I did to troubleshoot the problem:
First check that your battery fits tightly against the posts. You may need to insert part of a business card as a shim to snug it up.
May be a corrupt 3rd party app or setting
1. Backup with BB DM. Reload OS without any 3rd party apps. Also, do not restore any preferences, options or settings, just the important stuff like emails, SMS, calendar and address book. I did this several times and it would fix the problem for only about a day or two.
2. Copy all SD card files/folders to your laptop. Reformat the SD card and copy everything back.
3. May be a corrupt database. Create a free Gmail account and add it to your BB. Go into your gmail setup on your BB and tell it to sync contacts and calendar. Once all your contacts and calendar are on the web version of Gmail, turn off the sync on on your BB. Open DM and select Delete Data to delete all Calendar and Contact information. Once this is done, your address book and calendar should be empty. Next turn on the sync for both Contacts and Calendar in the gmail setup on your BB. This may take up to 30 minutes for everything to download. Once that is done, do a battery pull and *228. You can turn off the sync for both once everything is successfully restored, if you want.
You need to use Gmail to restore your contacts and calendar because it seems that backing up and restoring with DM just puts the same corrupt database back on your BB. Go figure?!?
Step 3. is what finally fixed my 9650. It seems I had a corrupt calendar. Hope this helps.