The trans does not always engage in any gear. Start the car it may just stay in neutral or it may go into R or D. Drives for a little while than disegages. Just revs up. Shut off the engine and restart ( sometimes a few times ) then it will engage.
It doesn't always seem like it goes back into low, like it is starting in D4.

Could this be electrical? Like the ECU or shift valve. It just doesn't seem mechanical.

Thanks...

 

Me and you are having the excact same problem p.m. Me when you figure it out.