How have you ruled out the clutch assembly including the clutch piston? Have you removed the transmission and inspected?
Well I bought an Oil head GS Saturday. Lacking room in my garage I wanted to put the Brick in my enclosed trailer. The thing was suddenly acting ok on the stand (wheels off the ground) . Shifted all gears fine.
Now I drove it about 4 miles home when this shifting problem started. I have a decent slope on my drive up to the garage.
So I fired it up and backed out of the garage to drive it into the trailer. It suddenly let it be known what failed (i suspect anyway) the unmistakeable banshee sceam of input shaft jumping clutch splines. 😳😔
I did the splines at around 40k (it’s north of 80k now). They had some wear but I hoped I caught it early enoughI was going to do them over the winter. 🤷🏻