955 overclocking

So, I have, as it says, a C3 stepping AMD Phenom II 955 Revision RB-C3. Many people have said they easily overclocked it to 4.0, but I can get it to 4.0 but never stable when running Prime95. So I use the MSI 790FX GD-70, and the 955 is running at 3.7. So could anybody guide me through on how to get it to 4.0. Also, i'm aware that it can be undervolted. How does that work? Please help.

Edit: Also, the weirdest thing happened. When i validated my 4.0 unstable in CPUz it said it was valid. But when I validated my stable 3.7 it said it was unvailid. Why is that?