[mr48chev]

It's been 35 and maybe more since I last worked on one of those.
Past experience says that they don't like being speed shifted worth a hoot. My 57 Panel with the extremely stout 327 had that unit in it and I got it hung up on a speed shift once or twice. Had to do exactly as you have been doing, stop and work the levers back to neutral and go again.

What I am wondering is if there may be enough wear or slack that there is enough slack that the the pawl is missing the arm and getting stuck between arms.

If you were to take the lid off and check the clearance between one or the other shift arms and the case I wonder if you could put a shim washer the right thickness to take up some of the slack but still leave enough clearance to work smooth.

Then the next question is what grease do you have in it? I'm thinking the last one I did I filled with white lithium grease. The last one being the 53 that my students and I did for a friend of mine and that now belongs to one of the students that built it.