If I hold down the Alt key while I drag the right indent, which I need to do
to specify an indent between ruler ticks, it then becomes ungrabbable by the
mouse. When I try to select it or click on it, Word thinks I'm trying to
place a Tab stop. This means once I position the right margin, it's stuck
there forever.

If I don't use Alt for precise positioning, I don't have this problem. Is
there a solution or work around to this?

Thanks,
Colin