Your problem is undoubtably in the vacuum control system. It is very likely that all you need is an adjustment. I just wrote an article on this subject that is in this months "Import Car" magazine. While the article covers many years and models of diesel transmission, the theory of operation is similar.
Before doing the adjustment the proper vacuum must be available and a working knowledge is handy. The article can be seen at: http://www.continentalimports.com/ser_ic20242.html