Just a couple basic things to try before we go further....
First, make sure that all your power saving settings are disabled (C1E, SpeedStep, etc...)
Next, set your FSB ratio to 1:1. Your board may or may not give you that option (your RAM has to be linked-which is another setting your board may not offer). If you're not sure, basically what that means is that your RAM is running at the same speed as your processor. So if you set your processor to 266MHz, then set your DRAM frequency to 533 MHz (this is confusing, I know...will explain later)