I am using a Saphire HD6770 with my Epson Powerlite Pro Cinema 6010. It works fine with 720p on the HDMI pot but with 1080p the screen frequently goes blank.
The mb bios and AMD catalyst are all at the latest revision.
that is probably because your hdmi doesnt have a high enough bitrate for 1080p and it is probably a low quality cable.
for good performance buy a hdmi cable which is made for 1080p like this one- http://www.newegg.com/Product/Product.aspx?Item=N82E16812189053