GOOD DAY,
Hello Gays I'have XFX Radeon 6870 Problem Is Screen Black After 40 To 30 Minutes I'have Checked System But Problem I Card.This Card Gave Me Customer For Repair.

Your system goes down, because your card received a thermal shock. At the plate it can be GPU or memory chips. To locate try detect by touch. If this is GPU, just install more effective cooler. If some memory chip, replace on the similar from another video card.

check fan(cooling system)..
refresh silicon
and check fan to work properly

thanks problam solvd whit silicon