CD drive problem? - inconsistent media check results
 

I've been trying to upgrade from FC4 to FC6. In doing so I managed to fry my old CD drive so I've replaced it with an older one. It seems to be working fine - if I boot into FC4 I can mount/unmount, see/read the contents, eject/close the tray etc.
However, when I boot from the CD to upgrade Fedora, Disc 1 passes the initial media check but no other discs will pass - even disc 1 if I retry it. When I initially tried to upgrade using the original CD drive, all 5 discs passed the media check.
If I ignore the media ceck failures and proceed to upgrade, it does the dependency check fine then hangs at the next stage (top shows a D status implying it's waiting on I/O).
Is this a problem with my CD drive, my installation CDs, or something else?
Ta