I ran into something similar over the weekend, with a s2735 variant (Intel 7501 Xeon development kit) I'm working on. I had assumed it was something I did wrong in Config.lb - I set the USB and IDE devices "on" even though my southbridge LPC setup disables them. When I set the USB and IDE devices "off" in Config.lb my problem went away. Not a solution, I know, but it might help track down the problem.
Steve