Pull the bulkhead connector and check for corrosion. Very common problem on old Mopars. When you move the wires to the resistor it moves the bulkhead wires and gets them to make contact for a while. I believe it is the light blue wire in particular that causes issues.
I would agree with this...
but, I was having the same problem plus charging problems on my 67.. NEW everything on car... Turned out to be the battery cable connection on the starter relay was loose.. Not floppy loose, more snug than tight kind of loose... Might check it..
