I’ve got a '99 Mitsubishi Pajero (which I think is a Montero in the US) 4x4 in Beijing China. When I drive, it pops out of gear, any gear, and when driving at any speed. The problem is that when I bring it to the repair place it never has the symptoms. My guess is that something goes bad when the car is warm and doesn’t go bad when it is cold, as when it’s at the repair station. I recently changed the transmission fluid hoping this was the solution. What could it be?
By your description it sounds like a manual transmission?
Whatever it is (maybe worn shift forks?) it’s inside the tranny, so it needs to come apart.