I've recently bought an Intel Xeon 5670, which I've overclocked to 4.2Ghz and when my system is under full load, the multiplier drops. I've turned off everything that should enable a dynamic multiplier, but it is still droping. For an explaination, see the link above.
In that blog post they are reflashing an Asus P6T Deluxe to an Asus P6T WS Pro bios.
And citing the following:
Please note the following steps not only do apply to the P6T Deluxe motherboard, but to P6T, P6T SE and P6T Deluxe V2 motherboards as well.