Overclocking a 512MB Gainward HD4850Golden

I purchased HD4850Golden a sample from Gainward (700MHz and 1100MHz frequencies). when I exceed the 700 MHz with the GPU (eg 705MHz) and I launch a 3DMark06, the first 2 tests passed without problems but as soon as the first test comes cpu: blue screen Is this normal? Do you have an idea to make my card stable above 700MHz?

Thanks

GAINWARD HD4850 Golden Sample brings a 700MHz core clock and 1000MHz memory clock, designed with higher efficiency heatpipe and special cover for better protection and air flow. It is equipped with 512MB DDR3 memory and 256bits memory bandwidth, supports PCI-express 2.0 interface and 800 units of stream processors.
Try to cool a bit more in your box and it should go

The memory got a boost from the reference 993MHz to 1100+ (effectively 2200+MHz). Although the memory isn’t running at maximum factory speed of 1200MHz (0.8ns), i still limited by 1100MHz. Catalyst Control Center allowed for overclocking the core to 750MHz (CCC driver limitation), but i didn’t manage to push the memory by more than 10MHz.