What's the PBA# of your board? Check Server Products — Important information on support of Intel� Xeon� Processor 5300. If it's lower than -801, there is no solution but to change the board or CPU, since it's a hardware limitation.
My PBA# is : D46952.702.
I realized a test on one of my server.
I made a downgrade of BIOS (via the following page: www.intel.com/support/motherboards/server/sb/CS-023585.htm) for the version S5000XAL.
Resul : no error message in BIOS, both processors are recognized. My server W2K8-R2 knows eight processors, as well as Sandra Sysoft.
Does it seem to you correct to think that the downgrade was benecial ?
Not really... Just the old BIOS is not "aware" of the limitation. The voltage regulators on the old board revision is not designed to support Xeon 5300 series. It may cause unexpected behavior, like system unstable, halt, reboot, even hardware failure.