I can try to explain part of your situation, and
that has to do with the viscosity of the trans fluid.
You said it works o.k. at first but troubles start
after being driven awhile. It takes 6-15 miles
for trans fluid to reach operating temp. When this
occurs, the viscosity becomes less, as with any oil.
If you have an internal leak, the fluid will stay and
go where it needs to when cold, but when hot it
will escape through the leak. The result would be
too low a pressure to get into gear. This is a
very general explanation but proven true with my 300sd.
It would not go into 4th when hot, either I had to keep
it in third or let it slip.
If you happen to decide that your trans is shot and
want to buy more time, put some lucas trans treatment
in, it is like 90weight transfluid. It took 3 quarts for
me but it did shift into 4th for 3000 miles until I was
able to replace the trans. I am not suggesting this to be a permanent cure.