Heater valves can get stuck in any position.
But:
By setting it to full high with the click in the setting or on full off, the power to the valves is interruped = valves are fully open= they move in case of your car.
So it is something in the control panel, or the temp sensor behind the control panel is out of order or completely dirty.
http://web.archive.org/web/200208130...t/acc_drt1.jpg
On my website I have about 52 pages IHKA training program for technicians, 2.09 MB, but it is in German.
Nevertheless, have a look at it
http://www.e38.org/e32/klimaautomatik.pdf