Dear Greg,
Thank you so much for your response.
I am NOT currently using proxies. I will use proxies, but I will eventually have to go back to the original 4k to assemble a final master output, which means working in 4k – also there are a number of FX shots which have to be created in 4k.
By the way, I have done all the usual – trashed prefs, reinstalled program, reinstalled system, run Disk Warrior, Disk Utility, prayed etc.
However, in principal, 4k should work ok in my configuration and gear, no?
Again, thank you for your time.
Harry.