Have you actually pulled the board out of the machine and checked for cracked solder joints on the pins?  Not impossible that the header pins are the problem.

Whilst the crimp pins may not have looked damaged, I can assure you that after 30 years they are not like new and should be replaced.  Did you notice whether the old pins were flattened compared to the new ones you put in?

My guess is that once you remove that board and inspect the back of the header pins closely you will find cracked joints which may well be your problem.