I keep getting Access Violation errors when attempting to move a table in a diagram. It does not happen every time, but it does happen often.
Check to see if your model is corrupt by...
1) Enable expert mode in Settings | Options | General.
2) Right click the model name in Physical Model Explorer and choose Test Model.
More at: http://blogs.inside.quest.com/modeling/2010/10/25/how-to-test-and-repair-your-model/