OK, guys, here we go again, one month later.

I did that MD5 test and it was OK.

I then bought new cd's as my first try was made on a cdrw which had some scratches...
I was glad to see that on the first try it booted very fast compared to the cdrw. But...after a while screen went black and computer didn't responded to any key inputs. Next try it again freezed already on the "Searching for Knoppix 6 in: dev/sr0" screen.
52x CD was burned at 4x speed.

/P