Your 3rd gear problem does sound like pressure leakage as the fluid gets hotter and thinner, piston seals or sealing rings.

The lack of a pressure reading at the accumulator while showing pressure elsewhere is mystifying.

The shaky gauge readings in R are probably due to the over-pressure relief valve (used after early '68) especially when modified by the B&M kit. It relieves pressure and then resets causing the needle to be unsteady.