Your PX-712SA is new? Where did you get it? :eek:
It could be a problem with SATA but you said that some discs work well and others don't. What's it like? Does it burn DVDs?
Please don't try the following at home unless you are sure that the drive works well with SATA, is not DOA and you have experience with such thing!
The Plextor PX-712SA is just a Plextor PX-712A with a converter.
If you open the back of the drive (secured with a pair of separate screws), you could remove the converter and use it as a normal PX-712A).
This voids warranty.