Macbook pro late 2011 screen dimming

I have a macbook pro 15 late 2011. It is still under AppleCare warranty.

One month ago the screen was blinking, with crazy lines and colors, i returned it and Apple promptly changed the logicboard and the screen.

But after that, i still encounter "screen dimming" problem. The screen dims and comes back to original screen brightness. It can happen for few seconds to up to 1 minute at a time. It happens "sometimes" when my computer wakes up from sleep, when i'm working on it, when it's on battery or it's plugged in. It happens few times per day. But the problem is not as obvious when i set my screen to higher brightness.

I took it back and the Apple technician couldn't see the problem after 3 days (probably he is not in front of the mbp for a long time). So i took it home and the dimming is still there.

Pls note that i already turned off "automatically adjust brightness". It's on the Mac OS X 10.8.3.

What should i do? If i take it back again and the technician is not able to "see" the dimming. I plan to tell him to set the screen to lower brightness hoping the dimming happens more often so that he can see it.

Please help

Thanks!