Probable cause: There might be a problem with upowerd polling information in conjunction with linux ati drivers.
There may be better ways to solve this problem, but to me the disturbance caused by upowerd appears to be very annoying.
So, I decided to remove upowerd from the system. At least on desktop systems this seems to be a minor disadvantage.
Usually, the demon is located at '/usr/lib/upower/upowerd'. Simply rename it to something like 'upowerd.old', restart your system. The problem should be solved by now.
Maybe, in future there will be a better version either of upowerd or the graphics driver.