After the next time it happens, check the device log for any errors, they will be hilighted in bold text. You may have to scroll down several entries to find the error. Hilight the error and click. If it's an app, try reinstalling the app, if the problem persists delete the app. If it's an OS problem, perhaps reinstalling/upgrading the OS will be necessary. If you don't already know, to view the device log, press and hold the alt key while typing lglg.
Posted from my CrackBerry at wapforums.crackberry.com
Grest and easiest advice to follow from star****e there, however, should that prove to provide little to no results or beyond your technical understanding, I'd say your next best play would be to just do a backup through DM then security wipe of handheld and finally a reload of OS via DM again.
Posted from my CrackBerry at wapforums.crackberry.com