Q: Late 2009 iMac Sierra freezing & incomplete shut down

Hi All,

I performed a Clean install of Sierra on my late 2009 iMac, 3.60 GHz Intel Core 2 Duo, 12GB RAM, 1TB drive, ATI Graphics. All seemed to go fine. I now have two problems:

1. At least twice a day the iMac will just freeze. The mouse moves, but cannot select anything. Cmd-Tab will not switch applications. Keyboard non-responsive. All I can say is that I always have Mail, Safari & Open Office 4.1.2 running. Have to hold the power button to switch off and re-boot.

2. At the end of the day, when I shut down, the process doesn't complete and I just get a black screen (Desktop is black, but the items on the Desktop are not visible), with the dock only. Again, the only way out appears to be to hold down the power button to switch off.

Anyone else having these issues? Is there anything I can do, save for a clean install of Yosemite which was running flawlessly before?

Many thanks in advance,

gimbers