Blop, did you buy a spindle of DVDs? maybe this is a media problem. The day my BTC 1004 was delivered, i went out to an office-supplies stores and bought 5 Verbatim DVD-RW disks, rated at "1-2x". When I tried to burn them with Nero, I was offered a 2x burn speed, but shortly after the beginning of the burn, the indicated burn speed would drop to 1.9, 1.8, right down to 1x. Firmware updates 0.43 -> 0.48 -> 0.49 -> 0.50 made no difference.
However, i went to a computer supply store and bought some Intenso DVD+RW and DVD-RW disks rated at 1x - 2.4x and these burn just fine at 2.4x.
What is the firmware version of your writer? What is the MID code of these Sony disks?