Old post I know, but..FYI
One of my drives was doing the exact same thing. Cable adjustments were fine. Was early in that season, and started to get worse. Pulled the drive apart and replaced the clutch. Definately was the problem, all good since.
The clutch showed very little signs of damage, but the cone face on one side of the clutch was flatter, not as sharp as the other side. Oil couldnt get out fast enough to let the clutch engage.
Probabally happens from engaging the drive in gear with to much RPM. Dont think I did it on those drives, as I bought them used earlier that season.
LE