rebuilt trans three years ago. Hardly driven it that much maybe 20000 miles. Has worked fine. Checked the fliud levels and put half a quart in it. That same night going down the freeway it was like it fell into neutral. Once it cooled off it engaged again briefly. Managed to get off the freeway but same thing happened again after just a minute ot two.

 

Not a good sign. Your trans is purely mechanical, so, you definitely have an issue. Drop the pan, see what you find.