Can you tell us which version of ME you are using? What JRE / JDK it is running with? See MyEclipse > Installation Summary > Installation Details for some of this info.
When it fails to open, what do you do to get it to work again? Do you restart MyEclipse or simply attempt to open the diagram again? When it does open, does the diagram look normal, as you last left it?
Sorry for the inconvenience caused.