OK, Let me tell you something about me. 1 and 1/2 year ago i bought a passat 91, it had exactly the same problem that your passat does. After looking for the reason of the problem finally i realased that in fact, this transmission failure is related with the electricall system because every time i turned on the antifog system (sorry for my english) of the rear glass it happened, and some times when the cooling fan turned on. Then i began to inspect the generator but it was working well. Finally i choose to replace the batery because when i bought the car it had not the right acumulator (i mean the battery wasn't the appropiate) then the failure dissapeared. Since then i have had the failure a couple times, but it is because the terminal connector is not fixed right.
May be your electricall system is overloaded or you need a bigger battery.
Well, i hope this tip help you.
C U