Just seeing this thread but yes, I also had this issue. In addition to not being able to send or receive messages for periods of time. Att was no help in my case either and was never able to pinpoint the issue
This is just happening to me this morning! My Lumia 900 is sending out a bunch of old texts this morning as if they were new today. All within the space of about a 1/2 hour. I did, about an hour before this, make setting changes to the email account I have on the phone. I hope that is not what triggered it? Any resolution?
Had this same exact thing happen to me with my WP7.8 Nokia Lumia 710. I had some text massages re-sent from about 5 months ago! I believe I was searching/reading/writing emails and sms when this occurred. I wonder what's causing this and whether it is preventable.