Its possible that your Power Supply is not supplying enough voltage to your GPU. Most people who do heavier overclocks typically have 650w Power Supplies or more. If this isn't the problem, then it may be that you need to send the maximum voltage that you can to your GPU, then overclock. Check to make sure you are sending the maximum voltage that you can in your overclocking software. Anyway, the Power Supply you mentioned isn't meant for serious overclocking, as 550w is usually just enough to run the system. Sorry for late response.
Edit: After rereading your post, it seems that you have run out of voltage to be sent to the GPU. Check your software again, but you may have to buy a new Power Supply to continue overclocking further. After all, overclocking is limited by how much power you can supply to the hardware.
Thanks for the response I haven't touch the voltage sorry for misunderstandings but this gen of video cards are really efficient my PSU could handle crossfire 7850 maybe not overclocked and crossiferd at same time also I overcooked from 860mhz to 1050 without a problem. I heard of many people getting this card oc to 1200 plus on a older version of gpu tweak ill give that a shot