Need Help with Overclocking Graphics Card

[speedy45]

I would like overclock my graphics card which is Diamond Radeon HD 5670 1GB PCI Express X16, and also here is the current Core & Memory Clock right now:

Core Clock: 850MHZ

Memory Clock: 1050MHZ

So here is the problem when I try to overclock it past the Core & Memory Clock. So using MSI Kombustor APP & using the burn in test for stability. I get artifacts when trying to overclock it unless I leave it at the current Overclock for Core & Memory Clock. So my question is there anything I could try overclock it?