Last weekend I noticed I was losing my data connection frequently when I was in an area without 4G coverage (the strength bars showed the "E" Edge network type).
That said, when I'm in an area with solid 4G, everything is fine.
The only cure seems to be to reboot the phone, and then it works fine for a few hours.
I used to run into this a lot but have not seen it since the Tango update. I also toggle to Edge as my highest speed if I'm in the one location I go to frequently that only gets ok Edge signal and next to nothing 4g which seems to help this issue.