help: wildly inconsistent cd-r diagnostic results

I just bought a plextor 12/10/32a cd-rw drive, and made 1 cd-rw disk (verbatim) and 1 cd-r disk (taiyo yuden) as an experiment. Then I used "cd-r diagnostic" program to test these 2 disks, along with a factory made CD (namely my MS win2k pro CD).

Here's the result: the original win2k pro CD is *ALWAYS* shown as a good CD with 0.00% of defect. however, the cd-rw and cd-r disks are flagged from being a good CD with 0.00% of defect, to good CD with 0.39% of defect, to a bad CD, with total randomness and inconsistency. Can someone explain to me why this is the case? Is this an indication of the poor quality of my cd-rw drive and/or cd-r/cd-rw media? Should I replace my drive pronto before I run out of the free exchange period?

Thanks for your help.