Mooney Electric Trim Problem

Perhaps someone here can suggest where to start looking for this
problem:
Ocassionally, the electric trim on my '78 Mooney 201 will stop working
for "nose up". When I press the button and push forward the trim will
run nose down, buy the other way - nothing. While I have the button
pressed, the manual trim wheel is difficult to move (like it in being
engaged by the electric motor), but when I let go the trim runs fine by
hand.

I have had this happen a couple of times now, and I am stumped. During
the preflight I run the trim full forward and back and it works fine.
At some point in the flight it goes out. Voltage on the bus is 13.9, so
there is no problem there (unless there is a huge drop at the motor).

Anyone here experienced something similar, or have any pointers?

Thanks
Ian
N201ZV @ LZU