I have the same issue from the last 4.4.2 update, when the phone is in standby for long time the data connection drop out, or time in time the phone scan the network and some time back to normal after some second or keep scanning and need reboot. when I check the network mode I found all option unselected, I select lte mode and the phone do not respond and have to reboot. I reinstalled 4.4.2 with kie and Odin but same issue . Samsung representative told me that the issue is from T-Mobile , that the phone is ok, and T-Mobile told me that the issue coming with the update from Samsung and affected to note 3 and s4. Who has the fault? my temporal free solution until the next update was replace the sim card in the T-Mobile store. I did yesterday and until now the phone have been ok with the data connection up all time. when it scan the network back to normal with not data drop out.
Last thing, go to wifi - advance option and I put disable (keep wifi on during sleep) and (always allow scanning) maybe these can avoid the scan the network time to time.