I'll have a stab in the dark and suggest that your firewire card may be responsible. You might want to try a different one and see if the situation improves.
I can tell you that I'm using a Sonnet Allegro Firewire card in my platinum G3 and it works fine with iTunes in both OS 9.2.2 and 10.2.6. I purchased the card from OWC for $38.00.