OPTIARC DVD-RW AD-7170A: Getting Error #6 when burning

I am trying to burn some DVDs on my Mac Pro in DVD Studio Pro 4.12 and some of them fail with "error #6 Unknown Device Error" and in the pop-up window it says the same followed by "(0x29 0x00)".

it's not every DVD but enough to make me annoyed. The detail on the drive from Sys Profiler is:
--------------
OPTIARC DVD RW AD-7170A:

Firmware Revision: 1.N8
Interconnect: ATAPI
Burn Support: Yes (Apple Shipping Drive)
Cache: 0 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RAM, -RW
Write Strategies: CD-TAO, CD-SAO, CD-Raw, DVD-DAO
Media: Insert media and refresh to show available burn speeds
------------

Has anyone else had this issue or have any suggestions. I've made about 8 coasters so far today and I've got a lot of DVDs to burn in the next week.

TIA