Popping out of gear is usually the result of wear on the end of the "cluster" gear. The gears do not have much meshing contact to begin with. When I had my trans rebuilt, the shop added groves to the gears to positively lock them in place and prevent the disengagement issue from reoccurring. Look for an old post of mine where I might have explained it better when my memory was more fresh.
When you push in the clutch and unload the gears, they release themselves from the groves and no added notchiness or other unwanted behavior is perceived.
