Going to sound daft - but which map version you running?
I had V3 with all sorts of problems - muted sound, outside digitised area, locking up, general crapness.
My V4 Disc overwrote the firmware automatically and it was like brand new ever since.
The Dealer supplied upgrade discs have the firmware update - if you buy a V4 disc that came with a new car then it doesnt have it - I got lucky.