It sounds like your problem may be caused by a few things.

1. A corrupt disk or iso file, or a cd/dvd burned at too high a speed. 4X is about right for both. Anything faster and it may fail to be read. The iso file can be checked with an MD5 utility. You can do a quick check to see if the list of files on the CD is complete by using the testcd command on boot up. It could also be bad media. I have seen numerous defective disks of different manufacture. It's a common issue.

2. The need to use the "knoppix dma" cheatcode at the "boot:" prompt during boot up.

Downloading by Bittorrent is the most successful way to download. FTP or HTTP will many times result in a corrupt file.

More information can be found through the "Documentation" link at the top of this page. Here are a few links to take you to some useful information.

http://www.knoppix.net/wiki/Downloading_FAQ

http://www.knoppix.net/wiki/Cheat_Codes