The FSB probably isn't set to where it should be since when you first put in a new CPU and mobo it defaults to like 100FSB until you manually set it for where it should be.
Find out what your FSB should be for that CPU and manually set it in the BIOS.
If it says your CPU is 266FSB then it should be set at 133, if it says 333 it should be set at 166, and 400 would = 200 however, I'd be surprised if yours was even 333/166 honestly.
Sometimes it's listed as 133 sometimes it's listed as 266 just realize that both numbers are essentially the same.
133 = 266
166 = 333
200 = 400
go to www.cpuid.com
and download CPU-Z and post a screenshot of the first Tab, that could also help us help you