Shadows crash Amd 6850

Hello there,

My 6850 becomes unresponsive after some time (differ from 5 mins to 30 mins)if i turn shadows on (basic or variance doesn't matter). I have to restart the pc.

i turn of post processing, vertical sync, AA-AF. resolution@1920x1080 .

When i turn off shadows and max everything AA x3 AF x16 post pro enabled, it doesn't crash.

Run packcheck, nothing.
AMD driver up to date.
Kombustor & OCCT Stability tests passed.
No problems in any games.

i5 760
4gb ram
6850
No overclock.

Actually the game loses it's soul w/o shadows.

Is there anyone with the same situation?

Thank you.