OK, now we are starting to communicate. Yea, that machine should squak by on the Knoppix requirements. I don't think I would just say it boots slowly, I would discribe it as booting slowly and never finsihing. Here are some things to try:

Cheat codes noscsi and/or acpi=off, as in knoppix noscsi, knoppix acpi=off or even knoppix noscsi acpi=off.

You say that you tried enabling DMA. Once again there is a key piece of information that I can't find in your posts: Which version of Knoppix is this? It matters because some versions have had DMA off by default bit some had DMA on by default and it needed to be disabled on some systems and some optical drives. I'll assume that you are talking about 4.0.2, in which case trying to enable DMA is the proper thing to do. But if it is an older version such as 3.7 then try the dma=off or nodma code. And try the DMA settings again with this different drive if you have not already done so.

It does not seem likely to be a CD issues at this point, but you might want to use the testcd cheat code anyway, just to see what this might tell you and to further rule out CD reading issues. And you might want to boot the CD and type in memtest (not knoppix memtest) to run a memory test, just to be on the safe side.