How much free memory do you have? If Options/Status/File Free is less than 10 000 000 bytes, then you're low on memory. Stoner's procedure can help you fix that: http://forums.crackberry.com/f3/how-...-memory-34633/
If you have plenty of free memory, then what's happening when the phone resets?
If you do, ensure all your pictures are actually being stored to the SD card. I had a friend who has had her BB for a month w/ a memory card and she kept telling me about the resetting. I then found out that the pics/ringtones were not being stored on the SD card. Another thing I did was deleted all her "SMS Outbox" and "SMS Inbox"