This is a frustrating problem. I have a 2010 27" iMac 3.06 GHz computer that I upgraded to 10.11.5 from 10.8.5 about 2 weeks ago. Since then I get the freeze about once a day. Mouse cursor moves but nothing else. Requires a hard shutdown to get the computer back up again. I stayed away from Mavericks, Yosemite, and El Capitan to avoid having to upgrade my Parallels app. Wish I'd stayed on Mt Lion cause this sucks