Comment # 26 on bug 76490 from Alex Deucher

(In reply to dex+fdobugzilla from comment #25)
> 
> Could this have something todo with the card having 4Gb of VRAM?

Doubtful.  More likely the card requires special some voltage tweaks for the
higher mclks.

Can you attach a copy of your vbios?

(as root)
(use lspci to get the bus id)
cd /sys/bus/pci/devices/<pci bus id>
echo 1 > rom
cat rom > /tmp/vbios.rom
echo 0 > rom