This is due to memory loss! Go to Options>Status, and check File Free status. You should have a minimum of 12,000,000 free bytes in order for your BB to function smoothly. If you're low on memory, follow the instructions on this thread to help you out.
After you finish with the memory optimization, make sure all your media items (e.g. pics, music, video etc.) are stored on a media card. When you lose memory, your BB will start to delete the least important items to free up memory. This includes texts, emails and call logs. Try to keep an amount above or around 12,000,000 free bytes.
Posted from my CrackBerry at wapforums.crackberry.com
Best thing to do is (1) delete what applications you don't need, (2) uninstall unnecessary themes, images, videos, (3) place photos, music, videos, etc. on micro sd card (4) set calendar and messages to delete items after 15 days, and (5) perform battery pulls.