Sandy bridge seems to be a bit of a bitch to OC with SS. You have to go in baby steps. do a 4GHz OC with 1.25vcore, then bump vcore to 1.35 and then 1.45 and then 1.55 etc. After than change multi to 50 and then whatever your max is
that should get it to what you need. You can have a play and find out what the easiest way is to reach max OC. I just have a profile now, load that, increase vcore and then load 5.7Ghz profile and im good to go every time