Radeon 6850 weird behavior

Hi,
For the last month i got a crazy problem and cloudn't solve it yet, i toke my pc to the PC lab but it's working there perfect.
one day suddenly my graphic card stop working, after a 3 days in the PC lab ( they update bios firmware & graphic card drivers ) they returned it and they say they didn't find any problem.

When i connect my pc at home i got BIOS post fails saying that graphic card failed, when i connect it in my work or at the PC lab it's working perfectly.

tried to check the power supply and reconnect the card to the PCI-E slot again with no luck.
when i connect a small graphic card like Geforce 6200 the pc working at my home also.

please advice i'm losing my mind here

i use antec full tower case
asus p8h67 mother board
sapphire 6850
corsair g.skill 4GB * 2