Something I think people need to quit confusing in their overclocks is that a 920 running at 20x200 = 4Ghz is NOT the same thing as 21x190 = 4Ghz with Turbo mode enabled. The higher multiplier from Turbo mode does not function in the same manner as actually selecting a higher multiplier. Turbo mode only engages when there are idle cores. When 1 or more cores are running idle it increases the speed of the cores that are under load. If however you have all 4 cores running under full load such as when you are running Prime95 then Turbo mode is not increasing the speed of the cores to 4ghz in the example above. You are at 3.8Ghz. And that is why if you disable Turbo mode and try running 20x200 instead your system most likely isn't Prime stable unless you increase the vCore more.