Sounds like the springs are weakening, I'm presently doing the doors on my 46 which is probably the same as your 40 and the latch spring on the passenger side is much stronger than the drivers side. I gave both mechanisms a good soak in WD40 and it seemed to cure the problem.Also check the leaver that goes from the inside door handle to the mechanism. they wear out as well and can cause the handles to sag.
BobT
