The LEDs used are LXML-PWN1-0120 which should be operated at a forward voltage of between 3.0V & 3.6V 350mA and have a stated 39000 hours of life.
The LEDS in the LM240WU6 panel are made by Lumileds which is a subsidiary of Philips who in turn produced the panel under license from LG. LEDs age, and as such the amount of light that is emitted reduces. I don't mean to offend anyone but you all seem to have missed the blatantly obvious and that is the LED backlight array. I think it makes the most sense and explains the cause of the problem well!
I would advice you to have a look at James Stuart comment/explanation below. Update (2018/09) My display is still up and running after changing the Capacitors, but I reckon this is only coinsidense.
