Unresponsive 2nd screen

I use a dual screen setup, clicking through thumbnails on 2nd screen, the main screen doesn't always update it's full size preview.
The same also happens when viewing adjustments.

I'm using 7.0.1/10.7.2/NVIDIA GeForce GTX 570 1280 MB/16gb RAM
Nothing shows up in Activity monitor to suggest a hang. Any ideas why?