Shifting in reverse problem

[tedm2]

I have a 86 Albemarle that has had a 290DP (and for now, briefly has a SP drive) that seems to have continual reverse shift problems every couple of years. With a new cable, shifting is fine for a year or two. It then works toward not shifting into reverse at all. This has happened with 2 different outdrives so I should be able to rule out internal outdrive issues. I would imagine that a shift cable lasts more than 2 years, and the last 2 cables probably had less than 60 engine hours on them before replacing. This has occured with cables installed by the dealer and by me. I have read and tried Ricardo's information on how to set the shift mechanism. Any ideas on what I may be missing?