No signal bars with GRPS/EDGE, and sometimes no service

Hi,

I have an iPhone that does not get any signal if 3G is deactivated. It goes either to "no service" or to no signal bars.

The carrier I have use two networks, and the phone chooses the best availble, but everytime it changes network the signal drops again. I can reset network settings, but it's not a permanent fix.

The weird thing is that sometimes when it says "3G" with no signal bars, i can still use internet. But i cannot make or recieve calls etc.

I was wondering if i could change the cellular antenna and that would work, or if this is a motherbord issue?