I'm using my GA-EP43-DS3L mobo with a Sapphire 4870x2 and 650W Croshair PSU. After about 1-15min of use my screen goes blank, some/all fans start spinning at full speed (not sure which ones, but I can definitely hear it) and I have to reboot. The monitor I use is the one I also use for my 360 and PS3, so I don't think that's the problem.
I wanted to update the BIOS since update F4 claims it "fixes VGA compatibility issues", but the problem is I never know when my screen is going to go blank, and I don't want the update process to be interrupted.
I am having the exact same issue with the exact same card. I also want to attempt to update, but am afraid to do so. My plan for today is to go to a computer place and have them put in an older PCIe card that hopefully works and then update the BIOS that way. Hopefully then I will be able to use my new 4870x2.
I got mine to work by flashing the BIOS through a USB drive. In order to do so, I went and bought a $25 PCIE card from a computer part store near me and then updated the BIOS that way. Now I don't have any issues with the 4870x2.