Screen going black - changed inverter. Brightness down and up helps

Hi,

I have a 13" Macbook A1181 here where the backlight goes off after running between some seconds to half an hour or so.

The LCD works as I can still see the image. It's just very dark when the light goes off.

Turning the brightness all the way down and then up again makes the light go on again. I noticed that it tends to stay on shorter when I power on the macbook in the morning and after some minutes it tends to stay on for longer.

I replaced the inverter board but that didn't help. Can I check the inverter board with a multimeter? Or what can I do to isolate the faulty part? Could it be the CCFL bulb? I read that the screen has a red tone when the bulb is about to die but mine doesn't show a reddish tone.

Twisting the display or closing and opening the lid doesn't reproduce the problem so I don't think it's the cable...

The problem exists for some months now and I use the macbook almost daily.