So you're saying you were not stable at stock, and had to overclock to 400FSB (remaining on the default MP of 8.5) to become stable? Something's is not right with your stock settings in that case - were you leaving everything on auto?

Perhaps your ram is running at too low a voltage/too tight timings, or the AUTO setting is making the boards PL (tRD) too low, when at stock. However unless you are manually setting everything to 'stock speeds & voltages' you can't really say that it is at stock settings.

When you have everything at stock, as you described it, using a combination of CPU-Z and memset, please state what your memory speed and timings are, and what the board's PL is. Also, what did you have the ram voltage set to?