Search the forums for memory leak. you'll find lots of info. When your BB gets too full it will delete emails and phone logs to make space. Try getting rid of pictures, mp3's, ringtones, unwanted themes, etc.
Thanks for the heads up. Call me boring but I don't have any pictures, MP3's or ringtones loaded on my Pearl but I'm sure it's a matter of low memory and I'll need to get more or delete something on the phone.