you may have got to the limit of your chip, not all can be pushed to double the speed, some have really high threshold, some, like it would seem yours, have a lower one.
It may be your motherboard too, if that isn't up to getting really good overclocks, it can limit to how far you can get your chip.
Have you tried raising the nb frequency? You don't have to push it far, i had to put mine up by a whole 5 to get my e6750 to 3.8GHz stable