Dell M6400 screen randomly dimming

My fiancee's Dell Precision M6400 has developed an interesting problem. Seemingly at random the display will dim. The OSD will show that the display is at full brightness. But she can hit the FN key to increase the brightness anyway. Sometimes it will immediately brighten; sometimes it takes several attempts to increase. And when it dims, the left side is noticeably dimmer than the right side. But when the brightness is increased to max, both sides appear to have the same brightness.

What do you think this is? Bad inverter, bad lamp or a misbehaving keyboard?

 

IIRC, there was an auto-brightness sensor beneath the screen which would adjust the backlight brightness. It was possible to switch it off in the BIOS (disable the auto brightness adjustment or something).

Though, given the description, I'm not sure if her problem is related to this sensor or there's something wrong with the backlight.