I would suspect the issue has to do with the two different display resolutions causing a syncing issue. Try running the external monitor at the same resolution as the laptop and see if the issue persists.
I have been using an external monitor as my primary monitor (with laptop lid closed) for a couple of months now with Win 7. This normally works very well, but occasionally my laptop puts the external monitor to sleep when I right-click on the desktop. I can restart the external monitor by opening and closing the laptop lid, but the right-click issue persists even after a reboot. I can even go into windows explorer, drill down to the desktop folder, then right click inside windows explorer and this shuts off my monitor also. I have checked all of the display settings and everything looks normal. My laptop is an HP G60-535DX, my monitor is an Acer AL1917W and the resolution is set at 1400x900 for the external monitor (recommended setting). My normal laptop resolution is 1366x768 (recommended setting). Everything works fine until I right-click on the desktop. Any ideas on why a right-click puts my external monitor to sleep?