I think it is a known problem, I had a similar one. With old versions of
the firmware the antenna could overheat I think? I had one fixed for
about £30 from fleet mobile communications, London (by post). Get them
to upgrade the firmware as well, and take a full backup of phone numbers
etc. before you send it.

If you google the archives you should find my post with the address, or
google for the web page with all the repairer addresses on it. Unless
you've got someone local, I'd go for Fleet - I got to speak to an
engineer and describe the problem, who said "Well we can't give a
definite price before it gets here, but it sounds like the antenna needs
reflowing, that'd be £30" whereas everywhere else I asked wouldn't give
me that detail of info.

Stuart