bc, you are scaring her. Plaster that is coming loose from the walls is usually just because the plaster keys that go over the lath when you put on the scratch coat have broken off. When a wall moves a little as the building shifts over the years, the keys snap and the plaster is then loose. Replacing the plaster fixes this. You can also apply screws with big washers to reaffix the wall to the studs. My feeling is that this is an acceptable repair on walls but ceilings can usually benefit by applying drywall.