I think it's a problem with the ambient light sensor.
I had this problem and to fix it you need to go into the advanced power options and there is an option for auto-brightness. If you turn this off then the problem goes away.
On the system tray, click on the Power icon, and then Advanced Power settings, and the option is towards the bottom of the list.
I'm not on Windows right now so not 100% sure about the exact path, but its something along those lines.