Xfx 6850 xxx edition, constantly crashing both games and stress

HI, i just bought this xFx Radeon HD 6850 video card, xxx edition, factory overclocked at 800/1050

I installed it last night, with catalyst 11.11 and played 2 hours of skyrim with apparently no problems.

This morning i added some graphical mods to skyrim and played it again.

After 10min of playing my pc frozen on a vertical bicolored lines screen.

tryed again and still goes on crashing, everytime with different colors.

I uninstalled all mods and tryed again, still crashing on both lines and full grey screens.

Then i started doing some stress test.

On both furmark and MSI Kombuster temperature goes to 63/67° in 40 secs and black sceen, frozen, have to reset.

I've done A LOT of tryes, i uninstalled and reinstalled about 4 versions of catalyst drivers, everytime happen the same thing.

I tryed both low and high Downclock on core and memory. It requires a little more time but crash the same way.

Temperatures go from 39° doing nothing to 62/65° while playing skyrim for some minutes.

My config is

Power supply CORSAIR 400W
MB ASRock M3A770DE
AMD PhenomII X2 Dual-Core 555 3.2GHz

Checked my entire configuration power demand and should be 288W min. and 338W recommended. That corsair psu is known to easily give even 450w.

With my previous video card, a fanless 9500GT, i never had this kind of problem, even playing skyrim and other highly demanding games.

Is this card possibly broken? anything i can try which doesn't compromise the warranty? i did anything wrong?
Thanks.