Andy, you're a godsend.I have spent some time discussing this with HTC and AT&T. Actually just got off the phone with HTC level 2.
I too have gone through all the options - new phone, new cover, new SIM card (you can just go into AT&T and ask for one, no charge), hard reset, everything.
From the information in this thread, and elsewhere I can say that the phone drops data on one of two things happening:-
- connection to tower is interrupted in current cell, but is re-acquired
- connection is handed off to adjacent tower as you travel to a new cell
Both these things will drop data IN 4G AREAS ONLY. If you are lucky enough to have consistent LTE in your area, you shouldn't have any problems.
Right now I'm working on getting the service menu codes so we can play with the radio settings. It sounds like HTC support reps have been forbidden to give these to end users on windows phone devices (I wonder why??!!).
If I get this information and it provides a fix for this issue, I will (obviously) post it, and I imagine HTC will as well. Here's hoping.....
Andy
Fwiw, after the LTE rollout, even a fallback to 4G in Detroit doesn't result in a data drop. I haven't needed to cycle the phone since the switchover.