To end with the G4 weird problem saga. I replaced the 40GB original hard drive with a 120GB SSD using one of the adapters recommended here. Installed OSX 10.5.8, which works acceptably well now (I think the SSD really helps make things feel more responsive and have faster starts). Installed MorphOS and everything works absolutely fine, with no weird problems. This really points fingers at the old hard drive, which was probably already failing, even though SMART didn't indicate so.

Now I can finally start playing with MorphOS to see if it works for me.

Thanks for all the help!!!!!