Try re-seating the cards again (let the machine cool down first). Clean all the metal contacts with alcohol or white spirit.
The other thing is the voodoo card may be getting too hot, have you replaced the fans on the voodoo card? as the ones sold with the card are not cool enough for the chips, these cards get very hot.
try wwww.overclockers.co.uk for cooler fans.
Is your power supply feeding enough power to the mediator board?
