Access 2003: Tab order screen acting weird

For some reason, adjusting the tab order using the tab order screen is
not working properly. When I select a field name, it selects *all*
the fields from the one I selected up to the top. I can unselect
fields from the bottom up, but if I try to select a field below the
selection, the selection is extended to the new fields. This makes it
very hard to adjust the tab order.
Is there an option somewhere that governs this? It didn't use to do
this, and I'd like to stop it.