Motor mounts degrade slowly over time and begin with noise/vibration.
I've never seen one get to the point where it moves a large degree totally without resistance - its typically more a matter of rubber tears.
I would also expect a motor mount to be consistent. Its smooth on lower rpm shifts? Where does the thunk come from?
I'm wondering about something in the clutch or drivetrain.
But int he end "maybe" but tell us more.
Grant
gee-lenahan-at-gee-mail-dot-com