I wish I had a better answer for you, but I don't. I've had this problem for a couple of weeks. When power downs and resets didn't work, I restored. Don't know for sure if it was the restore or the elimination of unknown, but specific, 3rd party apps. All is working OK, now.
Are you located in an area of poor GSM coverage? Are you jail broken? Are you unlocked?
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-Larry
I am on the iRovr EIC stream
16 GB iPhone 3G 
8 GB iPhone 2G
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