macbook pro early 2008 backlight problem

My macbook pro runs basicly good but occasionaly suffer from backlight problem in the past few years.

When problem happens, I just reboot and it goes back to normal. It happened many times and would always back to normal after rebooting so I did not worried a lot.

But recently the backlight problem came again and did not go back to normal no matter how many times I reboot. I also tried option+command+r+p and the problem remains the same.

It looks like the picture below:

Could anyone tell me how can I fix it.

Thanks