Sorry for my tardy response, I have been away for the past 3 weeks. Thanks for the replies, some useful ideas. What I have now done is switch the cable for the Mac to the screen to the HDMI from the display port, it seems to have fixed it........... so far. (two days), As was said these issues which have many possible causes are really messy, but the forum helps to sort these.
Thanks

Last edited by Rain; 08/18/16 01:22 PM.