I had this issue, and it appears to be related to a combination of two things:
The 32k SIM card and the v3.8 OS.
When I had this issue, I had the Cingular store replace my unit. I still have the same SIM chip, and the v3.8 OS and not have it come back.
However, I have noticed that the Bluetooth quality is starting to suck. I hear clicks during the conversation (this happens in the Bluetooth connection is encrypted or not). I haven't figured out what is exactly causing this.
Take a look at this post: http://www.blackberryforums.com/viewtopic.php?p=24549