I have my i7 870 OC to 4.0 ghz yet when I run 3d MarkVantage I get 10,000 under CPU score for the performance settings and weirdly enough 11,000 under the high settings.
(Overall Score (14182) Stock Intel Core i7 870)
Your CPU need to be reset to stock clock speeds then run your 3dMark score agian it should have a score somewhere around 14,182 for the stock Intel Core i7 870.
I would try:
1st: Turn off PC and take off side panel then remove battery off the motherboard and power cable off the PSU.
2nd: Then hold down the power button while the power plug is off and the battery is out.
3rd: Put everything back together and turn on, but don't change anything in the BIOS!
4th: Try to run 3dMark agian.
5th: Write down score or post on here.
6th: Restart computer and go into BIOS and start overclocking as long as your score is increasing you'll do fine.
When the score drops even by a little set it back down by however much you increased it.
I'm sorry if you don't see me post here much anymore, but I'm having a hard time figuring out this problem so I'm going to post on some other threads on tomshardware and if I here a solution to your poblem I will post it here! I hope I helped in some way and I hope the good people on tomshardware can get you all your questions answered Jarmel.