Delay when using the volume buttons

When I change the volume using the buttons on an Apple Wireless keyboard (with a Mac Mini) I almost have a huge delay between the key press and the action (up to several seconds and from time to time even several minutes). * using the slider on the menu works perfectly (without delay) * resetting the PRAM does not bring any improvement * I do not have any sound-related third party utility as boom (see an Apple Support discussion ) Any idea what the cause could be? **Edit** * Brightness is affected as well * I also noticed that the process loginwindow is not responding and uses 100% of one core * The console shows no errors