I eventually replaced motherboard
Purchased from where? Is it the exact same motherboard that was in there in the first place from the manufacture?
You posted that you have a Dell you did not state if the motherboard came form Dell or some place like Egghead or EBay. Just because you replaced the motherboard does not mean that the original drivers that came with original system will work properly if at all. How old is the motherboard (how long ago was it replaced). If it is still under warranty then send it back to be replaced.
Any time I have ever seen -
Physical Address. . . . . . . . . : FF-FF-FF-FF-FF-FF
Means bad, corrupt, improper driver or card.
Post #4 were you looking in the Device Manager or at the screen that came up after you inserted the Drivers and Utilities disk screen?