Underdog, the Combo drive was installed by Apple when the machine was assembled, so it is definitely supported. It seems like other people have had similar problems caused by IDE master/slave settings, so that is what I will try next.
wimmm, did the new cable do the trick?
Did changing the cable help?
I have the exact same problem with a 1 GHz Powermac G4 with a combo drive. Turning it on and off and on and off and removing and inserting the ribbon cables seem to help, but I have not figured out where the problem is. And my cables look good...