If it's like the later (early 70's) models, the transaxle counts on a spring loaded ball bearing pressing against a groove in the shifter rod to hold it in gear. Over time, normal wear smooths out the edge of the groove, reducing its holding power. The solution is likely to involve replacing that shifter rod if it's still available-which wouldn't surprise me- or getting a machine shop to fab one for you. You may also find that the gears are worn on the edges, producing a side load which pushes them out of mesh.