Acrobat XI Pro "Not Responding

I recently updated to Windows 8 Professional and Acrobat worked like a charm for 3 days with instantaneous reponse.

Now progressively I am back to the old problem that I had under Windows 7 and with Acrobat X and 9 - Whenever I open a new PDF it takes an extraordinary amount of time to open and I usually get the message "not responding" with an opaque window.

We have had this problem on 2 PC's.

Any ideas would be greatly appreciated.