[Thumper]

I understand the presumption that the clutch is involved since it is like the clutch is slipping transiently. But I wonder if the guide yoke(s) on 5th gear(s) is not fully engaging the dogs.

Does shifting into 5th feel like a deliberate "full throw"? If the rotating shifter guide that positions the yokes is not completely in the fifth gear position, the dogs could be incompletely engaged.

I am calling those guide yolks because I don't know the proper name for them. They moved transversely on certain gears on the counter rotating shaft, and drive shaft when you select a gear. Maybe get the rear wheel off the ground and manually go through the gears (shift with left hand while rocking the rear wheel back and forth with right hand so the dogs can align). See if it feels like a solid and complete chunk when it gets to each gear position, particularly between 4th and 5th. Do this with engine not running, of course.