Strange iBook Problem

[Joe]

Hello!

I have an 12" iBook 600 mhz running 10.2.2. When I move the screen to 
different positions it SOMETIMES dims to the lowest brightness setting 
which is pitch black. I know it isn't asleep because I hear the hard 
drive spinning. The only way I can brighten the screen is to close the 
lid entirely, put it to sleep, open it up, hit a key and it's back to 
the original brightness setting =O...What's weird about this is I can 
only duplicate (so far) the problem when the AC adaptor is plugged in! 
I have Applecare but this problem occurs (right now) less than 50% of 
the time. If I send it in they might not be able to duplicate it. I 
mean, it's not so much a big deal (yet), but since the iBook has been 
reliable as a Swiss Army Knife for a year, one tends to notice these 
things.

Any suggestions?

TIA,

Joe