HIS 3850 crashes freezes etc.

HI! im just new and my english can be bad, but i'll try to explain my problem

Asus p5k pro
c2d E8200
2x2gb PDP LLK
corsair hx 520W
win xp

month ago i boutght HIS HD 3850 IceQ 3 Turbo and since then i have problems, i have "gray screen" 1 time per day sometimes per 2 days, but its always the same and it mostly happen when i surfe on internet very seldom when i play a game, 3dmarks, memtest passed fine! no artifacts etc.

what i tried:
install clear system
tried different graphic drivers
turned off integred motherboard sound card  
installed new LAN card
set PCI frequency on 102 and 98

one thing yet, that specificationf of my computer is new, i had the same problems with
a64 3000+ s939
asus a8n-e

i thought that maybe is something wrong witch compatibility, but when i changed processor, motherboard and ram problem didnt disappear  

temperatures are ok maximum 70C for GPU

also i tried to set clocks lower than default but i had problems with that becouse atitool and atitrytool crashed system when i tried to set clocks, finaly rivatuner worked but ther was another problem, i had 70C for GPU in Idle   with lower clocks  



is there something i should try yet?


now i dont know what to do, i will try to give this card to service but i dont believe that will find the same problem becouse its seldom  .