Fog_Watch

You may very well have found the problem and answer, but you seem to refuse to accept the solution. Many disks that are burned at higher speeds may simply not work well in ALL optical drives. This may be the issue you encountered, especially with older hardware. Higher burn speeds seem to create some drive compatibility issues. It is encountered quite often with Audio CDs that don't play well, or at all in some CD players, but play well in others. Burning the disc at a slow speed usually resolves the compatability issue. Like it or not, it's a common problem.

Another issue could be that the drive/hardware is unable to work well with DMA acceleration or without it. The "knoppix nodma" cheatcode may help by disabling it, or "knoppix dma" to enable DMA acceleration. I'm not sure if 5.1.1 is enabled or disabled by default. You can try one or the other to see if that solves your problem.