Screen-backlight just randomly turns off

Hi Apple-community,

A couple of months do I have this problem. When I'm on my mac the backlight of the screen just randomly turns off. I could bring back the backlight by turning the brightness al the way down and then bring it back up, but after a couple of minutes or seconds sometime the screen goes backlight turns off again.

It's a MacBook White Mid 2009, 13-inch running OS X El Capitan. I noticed that everytime when I start up Google Chrome then the problem gets worse, but after while you're using one program the problem gets better.

I hope you can help me.

Aaron