So, similar to what you've said, these dips are probably going to be hard to avoid. You're using the firmware that was said to fix the issues, and funny enough you only have the one dip with B7W9 instead of the perpetually annoying repetitive dips, which is a worthwhile tradeoff. I don't like that the 4x CLV scan outright failed, but I would be more concerned if 4x CAV magically failed consistently.
Does the drive take some time to recognize DVD media? Does it have issues completing transfer rate tests? And do you have a way to try the drive on another PATA controller, just to check? Also also: can you force the PATA controller to use PIO mode instead of DMA to see if that affects anything?