If I have the brightness turned up the backlight goes out, why?

Recently I've been noticing the screen will dim a lot some times. So I use the keyboard commands to turn the brightness back up. But after doing this the backlight will go all the way out. The screen isn't going out, it's just the backlight. I can use an external light source and see the picture still showing. To get the backlight back on I have to put the Mac to sleep then when I wake it, I have to quickly turn the brightness all the way down or it will go out again. I've looked up so many things of why it could be doing this and how to fix it. So today I finally got a new inverter board to replace the old one. I thought this was the issue and this will be what fixes it. But after I replaced the board it still did the exact same thing. Is there someone out there who can tell me how to fix this? I am a computer technician so I can do it myself... But I need to know what's causing this problem. I don't want to have to replace my Late 2006 iMac.. I love this computer! So if anyone has any idea of what it's doing, Id love to know.