Russ, as you probably know I had this problem as well and did post on it. Have an update that may or not apply.
My trim would stick and then started running away. After 2 incidents I realized it wasn't a fluke. Local A&P (who is quite good and has worked on a variety of Mooneys) couldn't find anything.
1) Avionics shop found that the pitch servo had worn brushes. Worn brushes = dust in the servo. New brushes/rebuilt servo....runaway trim problem solved.....but...sticky trim still a problem. Trim would roll then stop, often requiring a forceful manual tug to get it going again. No pattern that I could identify.
2) A&P tearing his hair out, can't find anything. Calls out of the blue, he had a brainstorm remembering something from another aircraft years ago. Wires connecting the various yoke switches can chafe and wear. If the electric trim is operating while the yoke is moving the chafed wires then the trim can freeze. He demonstrated it on the ground. Went in and got all the wires out and several were chafed. All were replaced.
3) no more sticky trim!!
This may be a red herring rather than addressing your problem, but if all else has been tried see if this isn't a factor. You never know.
Robert