120 hp Merc I/O outdrive won't stay down when locked in lock down position

[redgoat]

My 46-year-old Mercruiser outdrive will raise up when put into reverse gear even though the lock latch is in the lock position. This happens at slow speed as that's about the only time I use reverse. The engine will raise up a bit as though it is in the unlock position. Is there a simple fix for this? Do I need a new spring or what? My outdrive is so old that it does not have power trim. It just has a hydraulic power tilt for shallow water and for trailering. Any suggestions?