leroydouglas, how do you suggest to really test this problem. From all I have experience and read in this thread this is a random issue that may take hours (hopefully even days) to happen at the most unexpected moments — the last one, I have just restarted, happened when I clicked on a hyperlink in Safari, the previous one, yesterday, when I was switching between one application to another, before that, I double click on an email in Apple Mail to open it… Both options, as much as they can stop it from happening, they do not allow for a full real life use. The main problem seems to be that Force Quit does not work at all, and even though the mouse will (at most times) continue moving and music will continue playing, no input or interaction will happen. This way, without the Force Quit window or even having access to open Activity Monitor, it becomes almost impossible to pinpoint the specific cause.