OK to begin with I am absolutely ignorant of the autotrans for the Opel but given that most auto tranny's work alike it probably does shift off of vaccume.
With that said if your car is running a little rougher than usual you may have aquired a rot out in your vaccume line. Most vehicles modulator lines run from the distributer but it is very possible the GT's comes from the intake manifold then runs under the car to the back of the tranny, Where at the back housing is located the Vaccume Modulator which when under proper vaccume tells the transmission when to shift ... usually when there is no vaccume the transmission will shift late and hard, in your case if the engine still idles and runs fine it is probably the Diaphram of the Modulator which is bad and causing your problems. Therefore the Modulator itself is in need of replacement ( Please keep in mind my experience with AT's is restricted to GM,FORD and Chrysler ) otherwise goodluck on fixing the problem ( Also as a small note i have seen underfilled AT's do this also)
The auto on my Kadett uses both a vacuum modulator and a cable for "kick down." If the cable breaks or sticks, the trans may shift prematurely based on not having a good throttle position input.
The vacuum modulator's on these do have a bit of a tendancy to fail, especially after 30 years or so. Check with Charles at Goin Manta, I know he has at least the one NOS one I sent him if you do need a new one.
The vacuum line runs from the manifold to the modulator on the rear passenger side of the transmission above the oil pan, and there is a little piece of rubber hose (@1 1/2") at the modulator end that likes to leak.
Fiat Trimatic's were a fairly common automatic transmission in many european cars of the 60's, 70's and even early 80's. As a result, they are usually not too difficult to get rebuilt. My local transmission shop just recently got me a new kick-down cable for the Kadett, and it was relativly inexpensiive, and Napa always has had filters and gaskets on hand.