Having done the tests referred to by Colin (and subsequent googling), I can confirm that this is the same bug as that reported in #36461 in xorg-server (Ubuntu) Xorg 7 has caused some serious problems for multi- head users, particularly it seems for those using onboard integrated graphics.
This bug should be closed, and linked to #36461 -- Desktop CD Fails with add-on graphics card https://launchpad.net/bugs/52939 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
