I am having the same problem with my '49 passenger side door. With me it was not being able to tighten the strike tight enough. The strike plate would work itself loose and I would be back slamming the door again. This weekend I decided to take the entire door off and replace the hinge pin, springs, etc. What I found was the hinge pin had worked its way half way out of the hinge which caused the door to sag to the rear. There are too many people on this board better able to explain "exactly" what to do to adjust your door, but basicly you need "good" parts to start with and tighten (loctite?) all fasteners so they don't move when it is adjusted properly.

Always get a 2nd opinion, especially if I am the source of the 1st opinion.