Turns out the drive had been defective from day-one, I just hadn't done enough reading of my own burned media to know. I called Plextor tech support, we ran through all sorts of troubleshooting, and in the end it was just plain bad.
Unfortunately it was 11 weeks out of warranty, so I'd have to pay $79.99 for the repair and $15 for the shipping. $95 for another 40/12/40a? No thanks. I even tried pleading my case to their RMA department, but they stuck to the rules.
In the end I ordered a Plextor Premium for $100 from New Egg, free 2nd Day UPS shipping. Installed it and all's wonderful.
Oh, also paid an extra $6 to extend the warranty another year, just in case
Dodeca, I did remove IAA and reinstalled my mmoard and chipset drivers. Unfortunately it made no difference.