I know this is an old thread but I wanted to offer some insight that I couldn't find online. That may help someone else. I was having the same problem and it got progressively worse, like it was the clutch dog. I opened the access cover to disconnect the shift linkage (previously pictured in this thread) for repairs. I found that the coupler you disconnect to drop the lower unit was loose. I readjusted and tightened it, now it's shifting and holding in gear better than it has since I've owned it. Save yourself some hassle and check this first.
