trying to put video board to other slots. Mediator too buggy with it some time. For my case, i can used video board correctly only after i resoldering all pci/chips/othersstuff contacts.
for first, try to put video card to first slot. run amiga, and just using scanpci or pciinfo tool. If you do will not have any devices attached to you mediator, so problem some deeply in hardware. If you will find that device connected to pci bus, but not displayed correctly params, try to use other slots or resoldering some mediator contacts (pci bus/etc).