Hum, if your CPUs are C0 (like mine) they should at least do FSB 400... strange.
You are right of course, these things are fast even at stock speeds. And 4s don't matter, but what does matter is the WCG performance. And there, every Mhz helps - that's why I keep pushing

You might wanna try FSB400 with 6x mult again (remember to disable EIST and C1E under CPU features for this) and run memtest from CD or USB stick. That way you'll see if it's the ram crapping out.