Sapphire HD 6870 OC not stable.

[Sensei]

Hi guys. I have a problem with OC my Sapphire HD 6870. So when i OC'ing my video card then system its freeze for few seconds and restarting.
Standard card clocks are 900/1050 in furmark temp is 90C(autofan settings) and its stable, but if I OC card core or card memory its unstable. I tried different clocks and voltages... Also custom fan settings.
OC is reasonable cause guys in reviews OC'ing that card at 1000/1200... I tried 940-980 core and 1100-1150 memory. Voltage on max 1.3. In games OC'ed card have freeze and restart\ driver stop working \bsod
When card is OC'ed and fan is fixed at 90% then temp in furmark is 80C(lower than with stock clocks)So i think its not overheat. PSU also good enought 850W
I tryed reinstall OS... didnt help... So its not a software problem...Different drivers versions also checked(no results) Any ideas?
System specs:
cpu - AMD Phenom II x6 1090T @4GHZ
mobo - Gigabyte GA-790XTA-UD4
mem - Kingston value ram 1333(stock)
hdd - WD Caviar black sata3 1 Tb
psu - Chieftec 850w(real 850w)
os - Win 7 x64

 

[gottistar]

My experience with OC,ing these cards , because i have 2 in CF, is they don't like to OC to far...i tryed every combo..i have mine at 950/1100....thats all i can do.....

Your PSU is fine, and reformat as you know wouldn't have done much.....