No Calls, Messages & inernet connection on 3G network

Hey, I got a iPhone 4s. As long as I stay in Edge Network (Edge Area or 3G dissabled) the phone is working great. As soon as I got 3G Signal I cant make calls or send SMS, and its not possible to call me. You cant use the internet connection too. When I disable my 3G to hit Edge network manuel it works again.

I switched (only) the mainboard to another phone to clear things up, and it gots the same problem. Another working mainboard is working great in the other 4s body. So the problem must be related to the mainboard itself, not an antenna or grounding problem of the body.

Any ideas what can cause the mainboard to act like discribed above? Should I head it in the oven for 10 mins on 190° Celsius in tinfoil?

Thanks in advance