I've noticed recently (certainly since upgrading to El Capitan) that occasionally I will be working with my MBP and switching between apps (clicking on the window) and nothing happens. So I click again, thinking it didn't register. Nothing again. I try to alt-Tab and still nothing. Can't pull up the Force Quit window via keystroke. There's no pinwheel to show a process is locked up. Following this. Sometimes the system comes back but sometimes it doesn't. Yesterday my MBP actually rebooted on it's own after one of these freezes. Has anyone else seen this? Have a suggestion?