In my experience
most shifting issues start when the external levers come loose on their studs. Soon the levers round the corners on the studs and then she will not hold an adjustment.
My solution for this is to remove the nuts and fill those gaps with Red loc-tite. The tricky part is to not let the locker wick up the studs and into the side-cover, which locks them right up.
My shifter was new in 1971, ............. it's been on a lotta different cars over the years.
