Have you got the newest voodoo drivers from Elbox for Voodoo 4 and Voodoo 5? That might be the problem.
Also, you could write directly to Elbox, they will happily help you with anything regarding Mediator. They have a nice support and a fast one too.
I have the problem with BlitzQuakeWOS GLQuake version for PPC. The 68k versions of GLQuake works really nice.
I've only got a Voodoo 3 2000, and MediatorPCI4000di. Good of you, that you take up subjects like this! Very nice.
My Amiga:
Amiga 4000 68060 50Mhz/PPC604e 233Mhz, Voodoo3 2000, DelfinaDSP, AriadneII, MultifaceIII, 66gb HD, 106mb RAM, 36x IDE CD-ROM
Regards,
Michal Bergseth, editor of Amitopia
url:
www.amitopia.tk