DVD burn error 0x80020022/"cannot write medium"

Hi! After upgrading with clean erase & installs from 10.3.9/iLife '05 to 10.4.6/iLife '06, I lost the ability to burn DVDs (data DVDs and movie DVDs) on my Dual 2.5 G5.

When I try to burn a data DVD, the error message I keep getting is 0x80020022 "Burning the disc failed because communication to the disc drive failed." The burn never even starts—no sound of disc spinning up, just text reading "writing track one" until the error dialogue appears.

When I try to burn a short movie with iDVD, I get all the way through the rendering processes, then in the burning phase the disc ejects and I get "The recording device reported the not ready error: Cannot write medium – incompatible format. (0x30, 0x05.)" Again, never heard any sounds of the drive spinning up/burning.

I've tried resetting permissions on the internal HD, restarts, logging in as another user, etc., but nothing seems to work. CDs still burn fine.

Prior to the system OS upgrade, I had burned DVDs successfully using iDVD 05, on the same blank media (Memorex and TDK disks, +Rs) that I'm trying to burn with now.

Here is my DVD drive info:

SONY DVD RW DW-U21A:

Firmware Revision: AADB
Interconnect: ATAPI
Burn Support: Yes (Apple Shipped/Supported)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RW, +R, +RW
Burn Underrun Protection CD: Yes
Burn Underrun Protection DVD: Yes
Write Strategies: CD-TAO, CD-SAO, CD-Raw, DVD-DAO
Media: No

Anyone have any suggestions? (If I should try a DVD drive cleaning kit, can anyone tell me where I'd purchase one, and recommend a brand?)

Thanks a whole wicked bunch!