xbmc won't sleep monitor in dual monitor setup

Hi all,

My media machine is a laptop with the TV being an external monitor. The TV is the primary monitor that has XBMC in fullscreen windowed, the laptop is the second monitor with the windows taskbar. The laptop lid is closed all the time. I use this setup so I can VNC into the second (laptop) screen and admin it as a windows desktop without interrupting XBMC playback.

I've set XBMC to turn off the display after 15 mins but it doesn't turn off. I've noticed that the screen flickers at 15 mins. It seems to try to turn the display off but doesn't.

Has anyone had any issues with sleeping the monitors on a dual setup?

Thanks,
Max