Ive had erratic data problems with XT and my iphone 4 since Ive been on xt (2 years)
It'll display at least 3 bars of signal strength and data will be connected/active but nothing will happen when i try to use anything that uses cellular data=open a safari page and just get a white screen.
If i leave it running it MAY start working and data will start flowing but not always. Sometimes i'll have to switch to flight mode then switch back or restart the phone.
My XT access point settings are correct (iphone nz ap)
I decided to switch to 2degrees for a few months and discovered that it worked fine, no issues. (voda fine too)
Ive tried restoring the phone etc. purchased another XT sim card for it but still the same patchy performance
When it does work it works well.