<VV> A reason to worry?

[Chuck McKinley]

Every so often while I'm driving my '63 Monza 900 coupe, I find that one of the doors has popped slightly open (~1/2 inch). It can occur to either door about equally. When I slam them shut, it seems to be OK for the moment. This just happened to my driver-side door when I made a sharp right turn at walking speed in a parking lot. I'm wondering if that's symptomatic of a dangerous level of frame flex. Is there a simple test I can make to decide whether this is a terminal condition such that I should finally retire at least the body of the car?
I can add that the floor pan is full of holes of various sizes and is patched several places with sheet metal pop-riveted through the original floor metal - not welded anywhere.
I just did a simple-minded trick of opening the door, standing on the rocker panel and basically jumping up and down several times. I did this on both sides of the car and nothing broke, for whatever that's worth. I weigh about 180 so it's probably not a very good stress test.
Advice is welcome, as always.

Chuck McKinley
'63 Monza 900 80 hp  3-speed