What BIOS are you using? I updated my BIOS to a new Beta BIOS about a month or two ago (when I got my Q6600). However, even on the old BIOS from late 2006 it still OCd the e6400 to 3.4ghz just fine. I just updated the BIOS because it woudln't recognize the quad-core until the BIOS update.
Also, perhaps it's just your RAM having issues with your motherboard or something else. Hopefully you get it worked out.