Brightness issue-automatic dimming

Hi everyone,
I just bought my 20' inch iMac 2 months ago. I've done some updates before but the last time that I did it was like 2 or 3 weeks ago.
I used to keep my brightness level at the maximum. I don't know why but suddenly and randomly my iMac monitor's brightness would automatically become dim all the way down now. I tried adjusting the brightness level in System Preferences > Display but fails. Every time I drag the level of brightness bar back to the right, it'll then automatically go down. Pressing the F2 key on the keyboard is also useless, there is simply no response. The only way to be able to keep the brightness level bar at the far right hand side (i.e. maximum) is to change the display resolution to a lower one and then bring it back up again to the highest. This is quite annoying.
It happens randomly. So sometimes I've been using the iMac for like a couple hours and all is fine. Sometimes this happens like 5 mins after I start it up. I really don't know what to do. Please could anyone kindly help?
Thanks a lot! I truly appreciate.

Mine does the exact opposite. I put my brightness at the lowest setting to keep the computer from getting so hot, and after the display goes to sleep, it is back at full brightness. I lower it either in the preferences or using the keystrokes. Still goes back to maximum.

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