Hello,
I do have similar problems but on an Intel GMA 950! An external minotor (22'' Samsung , 1680 x 1050) sometimes works, sometimes not and sometimes only at low resolutions. Clamshell mode sometimes with very strange results. Prpbably it even tried to display 1680x1050 on the internal(!) display (looks very strange).
Since a lot of users report similar problems on ATI and NVidia hardware I don't think this is a driver issue. Since clamshell mode is not common on standard windows machines, I think Windows itself is to stupid to do this.
I don't want to miss a big display so I only use windows in Prallels. It does everything I need.
Christian