I am not sure I fully understand what you are trying to describe. However if memory serves me:
The steering box is connected to the intermediate shaft with a "rag-joint". It has two shouldered bolts that hold it to the flange.
At the lower end of the intermediate shaft there is a bolt that goes accross the splines.
At the upper end of the intermediate shaft is the pot joint. There is a spring loaded clip that holds it together.
The other end of the pot joint has a "pinch clamp" that holds it to the splined end of the steering column.
I can't imagine the intermediate shaft "falling off" unless the plastic seal has been broken, allowing the intermediate shaft to collapse. You must be missing one of the above fasteners, allowing it to come apart.
Have you tried looking at another car? Most of these fasteners should interchange.
I hope this helps. If I am wrong, I guarantee someone will enlighten us both.
