It seems that it's not my EAH4670 card that CM does not like after all.
Oh no, it's Asus' OSD application that CM Shock Force hates.
OSD is an on the fly overclocking gizmo. Uninstall it & everything is ok.
Now if only I had not installed Catalyst 8.12, deinstalled Catalyst 8.12 installed Catalyst 8.2, deinstalled Catalyst 8.2, deinstalled the steam that I did not want that came with 8.2, reinstalled 8.12, switched on & off the relevant ASDS service, disabled & enabled the asus devices, installed open gl extension checker, ..... etc, etc.
My thanks to a Rocketman thread from 2006 that clued me in a bit.
Bye for now