AT&T and rogers, I barely use this phone, since I had it, only problems.
To double-down on what Shayne said, wipe SYSTEM as well, when jumping between AOSP/LG sources or JB and KK. Have you altered preferred network or APN at all? Leaving the defaults delivers the best stability if you're on AT&T. Finally, if you click settings--> mobile data --> menu, there are background data/auto-sync data options buried in there.
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