I had exactly the same problem that you describe (1984, 123 turbo). My temporary solution was the same, run it up in 3rd, slip it into neutral and then back into drive. THis can't be good for the transmission. My mechanic found that the bowden cable had rust in it and had to some extent seized. Fortunately all it needed was to be cleaned out and lubricated, about a 1hr job. This may not solve your problem but it may be a cheap fix that should be entertained before more expensive alternates are considered. Good Luck.