I have a 21.5 inch, mid 2011 iMac and recently it keeps freezing. The mouse will move around the screen but I cannot click on anything. The only way to clear it is to do a hard reboot by pressing the on/off button at the rear of the computer. It is particularly bad just after waking from sleep. I had a similar problem a year or two ago and the only thing that cleared it was a complete wipe of my Mac and then reinstalled everything, but I shouldn't need to do this every year or so? Weirdly, I have also had these random pink squares coming up on the screen (images attached) which would seem to suggest that it is a graphics issue.