Lots of people (including me) have had trouble with the PX-708A in combination with CD, CD-R and/or CD-Rw.
I suggest you e-mail plextor support. Their people who respond e-mails do have a clue and will give you an RMA number if necessary.
But the #1 option IMO would be to return it to the store and use the money to get yourself another burner, namely a NEC 2500, Pioneer A07, or Lite-on 811.
The #2 option would be to exchange it at the store. (This is what I did, and my replacement has been running perfectly for about 3 months now.)
If you've exhausted those options, then you can get yourself an RMA.