Is there a good chance there is some knob & spool going on here
...highly doubtful if you say your house was built in the 60’s. Most times the ground was the conduit or metal clad cable running between the fuse box and outlets. After many years of high humidity or dirty environments, those grounds can decay (no longer being of low resistance), as the conduit and metal clad cable connections start getting dirty, begin loosening, or begin oxidizing.