Start with this:
Menu>Options>Status If your free memory is less that 16mg this is most likely the problem.
First you need to make sure that you are not leaving programs running in the background. ALT+Esc will show what is running. If there are more than five you have programs in the background chewing up the memory. To avoid this always use MENU>end (or close) to end a program DO NOT use the red end key as this will just move it to the background and leave it running.
Do a hard-reset and check your free memory using the first sequence. If more
memory is needed then follow instructions in the link below: Why is my BlackBerry losing its call logs or message logs? - BlackBerryFAQ