iphone 3gs keeps saying no service. Tried all tips about resetting, know not sim or coverage issue. Any suggestions or is my phone done for?

Hi

My iphone 3gs keeps losing signal and searching and then saying no service, every so often I get signal for a bit in some areas.

I know not a coverage issue, my other phone works fine on same network, have tried a new sim from carrier with no joy, my sim works in other phones but none work in mine so must be my phone.

I've tried rebooting and resetting/restoring cellular network.

Any suggestions would be great, someone mentioned maybe an aerial problem??

Phone not under warranty and not due for upgrade and can't afford to replace so really need to try and fix if possible...