Ooooo, I know this one!
It's still the CD-ROM that's at fault, but possibly because of
location.
Move it to the other IDE channel. Switch from master to slave or
vice-versa, try the combos. one of my CD-ROM's would only work
properly if it was IDE1/slave and another only when as IDE1/master or
IDE0/slave.
Why? beats the hell out of me.