IMac (2008) display screen darker

My iMac boots and the moment the background wallpaper appears the level of light looks fine. However after 2 seconds or so the screen dims and becomes about 25% darker. The brightness is set to maximum. I experimented with colour profiles and tried calibrating one myself.
The brightness is consistent in all parts of the screen.
I don't know how to deal with it. I can't colour correct any images or videos because on another machines they will look too bright. I can't connect external screen either. My warranty expired 2 months ago. I'm desperate.

Did you try resetting PRAM?
http://support.apple.com/kb/HT1379
Some display-related settings are stored in PRAM.
If that does not help, as a test, try creating a new admin user account in System Preferences Accounts pane.
http://docs.info.apple.com/article.html?path=Mac/10.5/en/8235.html
Log out and log in to the new account. Does this problem with screen brightness recur in the new account?

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