Need help - no signal input after POST screen

[vanillatech]

After building a PC for a friends son last week, everything was fine for about five days when suddenly the display turned off and he got a "sync out of range" error. After rebooting, you see the POST screen and RAM count etc, then the screen cuts off and "sync out of range" appears again.

Phenom II 1090T Black Edition @ stock 3.2Ghz
Biostar TA890FXE mobo with 4 PCi-ex slots
8GB (2x4) Crucial DDR3 1600
Corsair 700W Modular
XFX 2GB 7850

First thing I did was check BIOS, nothing wrong there. So I flashed the bios to the latest version which fixes errors reading drives above 3TB. Still got Sync Out Of Range error after POST.

The PC works FINE with his old nVidia 8800GT card. Put the 7850 back in, and Sync error. I've cleaned all AMD drivers and reinstalled them. Reformatted his HD and fresh install of Windows 7 + all drivers, still didn't work. Tried running through HDMI, DVI, Displayport ... still nothing. I also tried a friends 6870 and guess what? SAME thing happens with the 6870, Sync error after POST.

Has ANYONE seen this before? What can I do now?

 

[vanillatech]

To add a little more info, I've made sure everything is connected properly twice, all power to motherboard etc. Changed PSU's twice, even tried with a brand new XFX 80+ SLI 650WATT. Still does the same thing. His PC works perfectly with either his 8800GT or a spare ATI 6570 I had lying around. But with either of the 7850 or 6870, after POST screen, we still lose all input signal.

Tried 3 different monitors, including an HDTV through HDMI and VGA.