You are aware that your 2 'Sony' drives are made by different companies? Your 510a is indeed a Sony drive while your 530a is a rebadged Optorite DD0401.

I really can't guess at what is going on with your bad burns, but it does sound like the classic bad media. It would be interesting to see some CDSpeed disc quality scans to see how the discs are burning. What media are you using? It could also be that the memorex drive (also not manufactured by memorex) just happens to like that media.

I just fail to see what Sony would gain by doing something like this.