There's no way for the cable to slip off the shifter in the outdrive, it's attached to the shift cable guide with a nut, the shift cable guide links the cable to the shifter, it is plastic and could be broken.
What is your idle speed at? Is your ESA working? Can you shift it easier if the engine is not running? Are you talking about the bellcrank lever slipping out of the shift fork?
Its Very Hard To Explain But , In The Outdrive Theres A Part That The Cable Moves That Pushes The Gear Shifter Up And Down ,well The Cable Keeps Popin Off That