Hi Terry,
I experienced a similar issue and found it to only occur on menus that I had created before I upgraded to Snow Leopard. In other words, I have had no issues editing menus that were created after the upgrade. If that is the case where you created some menus before your OS upgrade, the best thing to do would be to save your current menus as presents (both content and design) and then create new instances of the menus created beforehand.
Regards,
Brian