Scary possessed passenger door

it could be a worn latch or striker, new strikers are fairly cheap so i'd replace those at any rate. it could even be a sticky inner or outer door handle.

Since you have a convertible, I'm guessing your subframe isn't in optimal condition any more and that the car flexes a lot going over bumps, etc, adding to other issues and leading to the door unlatching. If you jack up one end of the car a bit, can you still open your door? If you can't, you have confirmed my suspicion. The stock early coupe/fastback structure is close enough to jello. I can't imagine what a convertible must be like.

Your hinges may need to be rebuilt, too (a much easier and cheaper job than adding chassis strengtheners). Check for this by lifting up on the door when it is open only a little way and seeing what happens If you see/feel much movement in the door before the car moves, the hinge bushings/pins need replacing and/or the hinges need to be tightened to the body or door. Also check for cracks around the door latch mechanism at the end of the door. This is a common stress area on old Mustangs, and they sell patches to screw in/bond in to repair the damage. Don't mask the problem by just adjusting the striker, though. This will lead to even more wear in parts actually causing the problem.
Daniel

I am willing to bet its a spring that has broken and when you are taking it out you dont realize that its missing. I know its a pain, but see if you can take both sides out and compare. I am pretty sure I had this happen years ago and replaced the spring with a $.35 hardware store spring.

Does it appear to be snuggly latched and then just kinda open up at random times?

I had that spring problem too. It is a pin to see and a pain to replace. I got new ones from NPD, I think. It helps to have a pic or at least a digram so you know where to hook it. Do a search; someone on here had the best pics I have ever seen of the assembly.

If I remember right, I pulled one out of another door that I had and compared them. But if nothing else you can pull the one out of your other door to compare (just mirrored)

i had a car dothat once and it was the door hinge bushings. replaced the bushings and pins and it worked fine