If you have a memory card in your BB, take the memory card out of your BB and see if the lagging/freezing stops.
If the problems stop when the memory card is out of your BB, you may have a corrupted file/filesystem on the card. Transfer the content off the card and onto your PC. Format the card in your BB, then use Mass Storage Mode to transfer your content back a little at a time. See if the BB starts rebooting, if not, transfer more to your card, keep repeating till all your content is back on the card. If you don't find a bad file, it was probably a corrupted filesystem, and it was fixed during the reformatting operation.
Kristina, my Curve 8320 was like that also. I deleted a few apps I didn't use, especially Facebook. Facebook is known to cause problems. It might also be that your memory is low. You might want to check that. Some BB users claim that if your memory goes below 12MB, your device might start to act up. Press and hold the ALT key and LEFT SHIFT key at the same time and then press the letter H. Check your free file memory.
Posted from my CrackBerry at wapforums.crackberry.com
