Laptop screen backlight usually doesn't work

Note: This question is very similar to Laptop monitor being very dark, no backlight?, but contains more information that hopefully might be useful.

Two years ago, I bought an Asus X83V laptop from a laptop repair guy. I got it cheap because it had a small flickering problem at the bottom of the screen. The guy offered to replace the screen for free if the flicker got worse. It did get worse soon, and he replaced the screen. I'd had no further problems with it until recently.

Two weeks ago, I went to tu on the laptop and I couldn't see anything on the screen. In the past, it had occasionally started up without the screen tuing on, in which case I would shut it down and tu it back on; it would then work fine. This time, it didn't work. After a few more tries, and restarting the computer entirely, I hooked it up to an exteal monitor, which worked fine. I also noticed that the picture was still showing on the screen, just extremely dim.

I contacted the guy who had sold it to me. He thought the inverter might not be working properly, and suggested flicking the bottom-center of the screen frame. Unfortunately, this didn't help. He doesn't live in the area anymore and also doesn't do laptop repairs, so I can't have him fix it for me.

I googled the problem and found a few possible causes:

• The lid switch is stuck. (Tried opening/closing several times, shaking the laptop slightly; nothing helped.)
• The backlight has gone bad.
• The inverter has gone bad.
• One of the cables powering the inverter has gone bad.

There seems to be no way to tell between the latter three except by replacing the different parts. This is pretty much how far the conversation has gotten on the aforementioned question. Now for the new information:

I noticed this past week that when the computer is first tued on, the screen is normal for 2-3 seconds before it dims. Also, if the computer is plugged into an exteal monitor, and the mouse is wiggled right after the screensaver shuts off, the screen works fine for 10-12 seconds before dimming. This behavior is consistent and reproducible.

Does anyone know why this might happen, and how I can fix it?

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