the sliding is caused because you froze the transformations. the blend is responding to the transformations of the original object vs. the transformations of the blend shape. if you freeze the transformations, then the relationship is gone (i.e. a 1 value will be the position of the blend shape.) never freeze transforms on the actual blend shapes, and there is no way to fix it once it is done. you can freeze the original object, but only before you create the blendshapes.
also, make sure that when creating the blendshape, local is on as opposed to world.