I learned this the hard way yesterday. (Namely by running around the building borrowing parts from co-workers and trying various combinations until finally hitting on the solution…):

When kernel debugging (KD or WinDbg) over Firewire (IEEE 1394), it is essential to have a high-quality cable.  The cable should not be any longer than necessary, and should use heavier gauge wire than those cheap cables that you get for free with a firewire card or camera.

Also, the speed of firewire debugging >> the speed of serial debugging, so in spite of the strict requirements, it’s very much worth the effort!