https://bugs.freedesktop.org/show_bug.cgi?id=76490
--- Comment #26 from Alex Deucher alexdeucher@gmail.com --- (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