I am having a similar problem with viewport locking, using AutoCAD LT 2008.
Basically the locking function doesn't "stick." Using properties, I set the scale and set display lock to Yes. If I double click the viewport to work in model space, zoom, pan, make changes, etc, then change back to paper space, the scale has changed, ever so slightly. When I go to properties, it is not at the scale I set, but now lists the scale as custom, with the display locked. In fact, I have to unlock the display in order to set the scale back.
Ruth Stadnik
Philadelphia, PA