Does your MOBO allow you to change the FSB speed? If so what is it set on? What memory DIMMs are you using? Are they rated for the speed you want? It may be that at least one of them isnt. If your BIOS settings are right (if you can set them), try removing one stick of RAM, then the other (if you have 2, if you have 1 then disregard. If you have 3 then repeat one more time lol).
If your BIOS is good, and your RAM is ok, you may have either a corrupt BIOS or a bad CPU. If you get to this point, look for a BIOS update. If that doesnt fix it, or you have the latest BIOS, it only leaves either the CPU or a faulty MOBO.