Parsimony still suggests the shift cable adjustment. I would particularly suspect that from it happening
docking, because human beings tend to shift in that situation while the
engine RPMs are still high.
Suggestion: First, make sure that all the attachments to the cable are tight, because slip rather than adjustment could be the problem. Then, disconnect the cable at the
engine, shift into forward, N, reverse and note the position of the arm in each case. Reattach the cable, and have someone shift at the
helm whille you watch. Did the arm go firmly to the three positions, or is it marginal between N and reverse?
There are lots of other nasty possiblities, but do remove that simple one from consideration before you hemorrhage time,
money, and greasy hands.