I am currently in contact with Apple concerning this and as of this present time, they are being helpful, but it is clear to me that they are as yet unwilling to admit there is a problem. It appears to have all the signs of a memory leak. The imac will freeze at any given point with no warning examples with multiple apps open or just one or even when computer is idle with nothing on the surface open. Suggestions were to screen grab activity monitor, however rather difficult when the whole computer has frozen. Other suggestion run in safe mode, not helpful as I cannot use various items such ext Mic as it does not recognize it. I am beginning to feel I am going round in circles, I also notice that applications take longer to open than previous version The reason for upgrade was to remove that annoying issue with wifi dropping off for no reason wish I had never upgraded, that was a minor issue compared with this. Now the computer is as reliable as a PC very sad!