W660i is not detecting network?

My sony ericsson w660i is not working for network.
All other functions are working properly.
It will search network for some time and give display as no network found.
If i search manually for network then it will search on without stopping.
And also when i select only GSM option the mobile hangs and i can restart only through removing battery.
The phone was working fine for 13 months after that in one fine morning it stopped.
I gave phone to service centre and they tried to load software and it failed.
So they gave me back and told that it is hardware problem and cannot be rectified.
I gave phone to another service centre and that person identified an error while flashing.
He rectified it and found that display was not working.
So he changed the display and everything came up but network was not at all detecting.
He gave me back the phone and told that he is unable to rectify this network issue.
Could you please tell me that how could i proceed this?
I like this phone very well if it can be solved then i would be happy