iPhone 5 odd signal problem

Hi to all ifixers, recently I bought a iphone 5 (european gsm version) affected by some strange reception problem;the phone reads sim cards, can find a network with full signal strenght, stays right that for 2-3 minutes than signal instantly drops to one dot and finally disconnects itself from network showing "no service". If I leave it alone after some minutes (5 or more) the phone usually find the network again and the same situation repeat itself. I thought in the first place of an antenna problem, I tried swapping with two others charge port assembly, changing interconnect cable, restoring normally with itunes in and dfu mode,resetting network settings, swapping sim card with no luck. Then I went inspecting the motherboard with a magnifying glass not being able to find any components missing (at least from what my entry-level instruments allow to), lower and upper antenna seems intact,main board did not suffer from any apparent damage. The only thing that I found not right was the non magnetized screw that was indeed magnetized!. I swapped with the right one right after. One thing that I noticed was that looking in the settings menu a moment before the phone loses its network the carrier field goes blank. Any advice on the possible fault?