OK, I have solved it. After a lot of swapping cards, changing the PSU (it was suggested the 450w unit may not be powerful enough, but the 600w replacement made no difference. i left it in anyway.) I tried different combinations of settings in the CCC. There is a setting in there called "Compuesto" (I think that means compound in english). Ticking this box has solved the problem.
I have no idea what that setting does or why it should work but it has.