If you are using Lightroom 6, then turn off the Graphics Processor from Lightroom Preferences.
Go to Lightroom Preferences from the Edit menu
Click on Performance tab
Uncheck the Graphics Processor
Let us know if that helps.
I am no longer using lightroom 6. I was, but in an attempt to fix the problem, updated the software (which forced me onto CC). I have had a look at what you have suggested and the graphics processor is already disabled.
This did the trick - thanks so much!
Its amazing what the preferences file will mess up when it is corrupt. Adobe needs to spend a little coding/debugging time to make the preference file more robust.