The rod adjustment might be the problem. I know that Chevy trucks from the 60s had problems with gears wearing and this would cause them to pop out of gear while driving.

The solution was to shift into third, the gear that saw the most wear and resulted in most of the pop outs, and rest your hand on the end of the shift lever as you drove. You starred with your left hand.

The added weight of your hand on the end of the shifter was enough to keep the thing in gear. This assumes you had a three speed with the shifter on the column.

And that is your high tech solution for the day!