[willy]

NC had the same problem a couple of years back, sometimes because of vibration, wear or the Gods of the Sea the shift rod that goes down the shaft, which is adjustable with a threaded nut and a lock nut backs itself out far enough to just barely not engage.
Something to check your self, I had it happen after a water pump replacement job in which I apparently did not adjust it right and it would go into reverse but as soon as you gave it any gas it popped out not engaging fully.