I got similar issues on Pegasos 1 with MorpOS 3.06 and Voodoo 3 3000 PCI 16 bit 8 bit too and usually it fails to change screen resolution differently from 1024x768, ever over that res. Tried it on 3 different LCD moniors.

Then I got black screen on a new ATI 9250 128 MB PCI! But this video card can run fine on a PC Windows.

My Mini G4 1,5 GHz looks working at 16 bit.