Hmmm, all good disc brands.
Any chance your 3550A could be going South?
I have an idea. In the Nero 9 Toolkit, you'll find DiscSpeed. Can you do a "Create Disc" with it and post a screenshot of the burn graph when it's done (little floppy-shaped icon in the top right of the window. Save as a .png file).
Also, if you could run a Burst Rate test (with a burned disc in the drive) in DiscSpeed, that would help rule out another problem too (DMA) - with the Benchmark tab open, go to Run Test -> Burst rate. Then post the result.