Knoppix either stalls or take ages to boot up completely

I bought a CD with Knoppix version 3.7 to run on my PC (I have
used DOS years ago but now, really only know Windows). The
CD booted up but when it reached the line relating to 'creating
/etc/fstab', it hung/stalled and went no further.
I rebooted but the problem persisted so I telephoned the shop
from where I bought the CD and the person there said this did
happen 'occasionally' and I should press f3 as it booted up
and then type knoppix nodma

I did this and it was fine reaching the shell/menu (doing so in
about 2 minutes in total, i.e., my PC is normally fairly fast)
However, after this welcome success, when I booted it up again
later in the day, I again typed in knoppix nodma and it now
takes a-g-e-s to boot up completely (4-5 minutes at least).
It seems to now wait on the USB line and then before
reaching the main Shell menu.

In fact it takes so long it almost seems as if its hung although the
CD drive light is on so I know it is still working. The only way I
can now get to the shell within a reasonable time is now typing in
failsave when booting up.
I cannot understand why its now taking so long and why this has
suddenly started happening. Can anyone help please?'.

If you burnt your CD at high speed you might try burning another one at a slow speed like 4x and see if that improves the computer's ability to read it at boot time without a lot of rereads.

I did not burn it - I bought it already burned from debianshop.com. I have done a testcd and
the disc is OK
Can anyone help me please?