swhile: It appears you have just discovered the EIST feature (Enhanced INTEL Speedstep) which throttles your CPU's speed down when it's not under load (it lowers the chips multi to x6), as soon as you put any load on the system the speeds will increase back to normal.
Download Core Temp and run it, you should see a STAT called VID (guessing it will say 1.35vCore). Manually set the voltage of your E6600 in the BIOS to the same value as the VID and start testing.
Using the Stock cooler is fine when doing smaller overclocks but once you start having to add more processor voltage (vCore) and really increasing the chips frequency you will find the stock cooler starts to struggle.
Using Core temp to keep an eye of things you should stop overclocking further once temps hit about 65c full load.