FX5700 Ultra TDH Overclocking

Okay. So I'm taking another look at OCing my FX5700U. The card I have is actually the Leadtek WinFast a360 Ultra TDH, which is the same as an FX5700 with DDR3 RAM, which it does have. The stock speeds of this card are 475/950, but I've only been abled to OC it to 530/1.06 without seeing artifacts. I've seen a few other people with the same card and they have great OCing results, and other people with differant branded 5700Us and they even get better OCing results. How come my card is being held back so much? I'm using the Arctic NV Silencer 2 by the way, and I'm not seeing any problems with RAM.

It's just like a cpu,some overclock better than others.