volume on iMac increases by itself

For about a year now the volume on my iMac desktop increases itself every few minutes. We lower the sound with the keyboard buttons--this only works occasionally, and with the slider in the sound utility and with the slider on the menu bar. These usually work for a minute or two, and then the volume creeps back up. I have read that some apps will trigger this--well, it happens when we use Safari. All other apps are closed--no one has suggested what Apps might cause the problem. I see in the comments this issue goes way back to 2007. It is soooooo frustrating, and apparently pretty common based on the questions asked on various Mac forums. I am running 12.4 (the upgrade changed nothing) on a 3.5GHz Quad-Core Intel Core i5 and start up with the Mac HD. No peripherals attached except an external backup drive by LaCie. HELP!