Francisco,

        It sounds like the problem with your transmission is that something is
causing it to not want to shift gears properly.  The cause of this is
probably low oil pressure to the clutch assemblies.  Be sure to check
the automatic transmission fluid level.  If it is very low, this can
happen.  However, what is most likely, IMHO and experience, is that
there is a problem with the accumulator piston either having failed or
perhaps not operating properly because of something causing it to hang
up.  Look at the Bently book, page 38.25 to see the location of this
part.  You can remove the transmission oil pan and valve body to get at
this part and remove it--I've done it.  The part cost about $35.00 the
last time I needed one.  This part is common to all the auto
transmissions from VW or Audi that were used in the '80s.  A common
failure of this part leads to the transmission not wanting to go into
reverse, or that the transmission doesn't want to shift until warmed up.

I'll make a guess here, and since you say your transmission was rebuilt
15k miles ago, and say that this is probably the most likely problem
with it.  Just remove it, inspect it, clean it or replace it if
necesary, use a spray cleaner to clean the  cylinder where it operates
well, and put it all back together, and you will have fixed the problem.

Good luck,

Chuck Hill