I have found on other forums that this can be caused by the board connection becoming unseated. You will have to dismantle your phone, clean and reseat the connections.
I have a similar problem, but haven't yet tried this, so can't confirm.
The other suggestions I have seen (and recommended by Sony Support) haven't worked - master reset and 'download the latest patch software' (I already had it).
Hope this helps someone.
KevinT, November 2007
