OK, with thanks to people on the Adobe Forum and our very own Artobest, I think the problem lies with the checking of the 'Use Graphics Processor' in Preferences>Performance. If UGP is checked, and then when the Crop tool is used, 'Delete Cropped Pixels' is NOT selected, rotation with the Crop tool leads to the patterning. If UGP is not selected, and PS6 then restarted, it does not matter whether 'Delete Cropped Pixels' is selected or not.
I would be very grateful if people so far unaffected by this problem and still following this thread would attempt replication under these conditions.