My 1985 740 turbo was doing the same thing on position 3 or 4. I changed the negative battery cable and the ignition switch and that problem is gone. If your vacuum servos goof up then you can have no air coming out. Mine still does that in the high position because of a vacuum leak but blows strong in the normal a/c position. Take a look at the voltage reading when you put it on high. You should only get a very tiny dip in voltage with each click as you crank up the fan. By the way, I had already changed my plus battery cable.
Incidently that ignition switch is real easy to change. You may not have both problem, but I would check the ground cables including the one on the alternator.

Randy