12.1 rc2 radeon 6790 boot issue - fails to reach desktop

asus m5a97 pro 4gb ddr3 amd athlon 2 x4 630 radeon hd 6790

When the livecd boots it loads to the point of a scrambled image which resembles the windows wallpaper (colors all scrambled)
I have tried nomodeset but no joy any other suggestions would be welcome.

gpu runs 100% fine under windows and can load pre 3 kernel distros without issue
current fails are ubuntu 11.10, opensuse 12.1 rc2, fedora 16 all give the same result!


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