I sure hope 4.2.2 is going to help somewhat.
I have the exact same issue, as soon as I use any app that requires data connection it will disappear for a few seconds, long enough most of the time to have the app or page to time out. I am still on stock everything, hesitant to try anything so far.
I do use WIFI extensively, one thing I have noticed though, rebooting the phone every other day somehow helps,
I am on TMO in houston metro area. There are some dark spots on TMO network all around houston, I also tried the fast tmo (TMO LTE?) apn and did have some success with that.
I have seen posts elsewhere that a replacement device was required to fix the issue, anyone got a replacement device that completely eliminated this issue?
I blame TMO most of the time though not the phone, I had similar troubles (much worse) on my SGS4g as well.