iPhoto freezes when creating shared photo stream; what to do?

iPhoto 9.4.2 freezes every time it attempts to update a shared Photo Stream, even when it does it automatically upon opening the app.

Have tried re-installing iPhoto, repairing iPhoto library permissions and database, all to no avail. Have managed to delete the shared photo stream and redo it, but no joy; it still freezes when iPhoto opened.

Suggestions?