There could be a couple of things here...
1) The battery might be sitting loose in it's compartment. Some people (although minimal) have tad play in them but for majority it is tight. I have heard people having play in the battery compartment and it rebooted about twice a day. You can try a bit of facial tissue jammed in opposite the battery terminals to prohibit it.
2) Issue with microSD. If you have a microSD, try reformatting or even having your berry without the microSD card for few days (just to notice the behavioural pattern)
3) Issues with your OS. I would recommend upgrade the latest OS for 8330. Follow the link below to understand the process of upgrading your OS. http://www.blackberryforums.com/gene...ilanguage.html