We had the Same Problem
Tried Almost all Firmwares
other Disk(and yes, some did on 4x like the pioneer who costs almost 5 Euro, but that was too expensive)
Contact Sony, several times
Conclusion: There was no Solution for our problem, it is like buying a Porche that can only drive 90 Km/h
So last week, we sold the sony and bought the pioneer A06 who is cheaper and .. it has burned every disk already on 4.1...(we used Princo's)
I think my conclusion is clear and if you read forums on the net... I am not the only one who thinks that way..