It could be the synchros or a bad shift cable, but usually the shift cable would be noticable in several gears, like 1st-3rd-5th or 2nd-4th-reverse. Considering your description of the symptoms I would lean more toward a dead synchro. It won't cause the gears to pop out while driving but it is usually very annoying. I drove a car with a dead 1st synchro for 50k and it never got any worse

You could have the trans rebuilt, replaced, or just live with it