I was getting the dirty disc error occasionally, then it progressively got worse and the machine was unable to write, then read discs. I pulled the burner out, cleaned the lens, reinstalled and it worked for one or two discs, then failed again. It seems this is a common problem with this machine.
As a last resort I found a sony dru-11a burner and installed that into the machine to replace the DW-11A. Some customization is required to get it to fit. The machine works great now.
Note to Sony:
When I bought this thing originally it was an expensive machine. I've been an early adopter of technology. I've come to expect a level of quality with the Sony technology and for the most part am happy. This machine however seems to be a "dud". Lots of people on the web complaining about similar problems and the cost of the replacement drive (DW-U11A) is extreme. I doubt that I'll buy further Sony equipment.
Tom Stephenson, September 2009