For the last 15 weeks, this iMac has been freezing sporadically. I have run out of ideas, and hope that 10.11.6 fixes the bug.
DriveDx
Two free tools to check how long your SSD, including those in Fusion Drives, is likely to last.
How long will the SSD in your Mac last before it fails? Some seem to be going to an early grave. Is yours?
What do you do when your Mac seems to have restarted quietly when it should have been asleep?
How much is the data stored on your Mac’s drives worth? DriveDx is a utility which could save you from that loss.
The Eclectic Light Company 