After flashing a build of CM11 back in February, I've had problems with the data-connection (2G/3G/H, etc.). It works when starting the phone, but suddenly it just dies. It looks like it's working, but no data seems to be transferred. I have to turn data off for a minute or five, and back on. The it usually works for a while again, until it dies...again..
I've fullwiped (except SDcard and internal SDcard), and tried several modems. Then I went from CM back to Ultima ROM (based on stock Samsung), and used it for a couple of days. Data worked perfectly. Thinking it solved the problem, I tried Omni again, after wiping/formatting data, system, cache and Dalvik. Installed it, data seems to work, but after a few hours, it just dies again.. The signal icon still has the H+ connection, etc, but I can't use it. Just like it's offline.