I have fully updated CC. when using the adjustment brush in ACR, if I move a slider such as 'Exposure' or another one, the preview picture breaks up into large multiple squares while the slider is being moved.
Any ideas or solution to this?
Try to disable the GPU.
While in the Camera Raw window, press Ctrl+K (Cmd+K on a Mac) to bring up preferences.
Then uncheck Use graphics processor at the bottom.
Thanks - that seems to have sorted the issue but does that mean I have sacrificed performance elsewhere in PS and ACR?
The GPU setting for ACR doesn't affect Photoshop – ACR is a plugin.
I don't use ACR, but in Ligthroom (in which ACR is integrated), the most important advantage of the GPU is that it will improve performance if you have a 4k or 5k monitor.
Updating your graphics driver may make it possible to work with the GPU enabled.
That's helpful. I always keep my GPU driver up to date so it might fix next time.