Since two governors exhibit similar responses, the problem has to be either the oil supply to the governor or a mechanical jamming of the blades.
What stands out for me is that the problem disappears after a few moments or landing - when the engine/prop rpm and load has reduced - and seems to reappear after the engine/prop has been constant or at least loaded for a while?
I once had a car auto gearbox that after driving for a while would fail to select gears, but if I stopped and idled the car in neutral for a few minutes, the gearbox worked again.
Is it possible to fix an oil pressure gauge to your governor oil supply? And also an oil temperature gauge?
Then, although you say the blades and pitch mechanism move, do they really move freely or could they be mechanically jamming at certain rpms? Is it possible to remove any springs etc and feel the pitch movement of each blade and the governor - moving them by hand - to feel if they move smoothly throughout their range or have crunching or snatching at any point?
I am leaning towards a problem with the oil supply, (I take it you have double checked the oil level and correct quantity?). Another thing you might check is the balance of the prop.