I believe cancerman is probably correct about the bad ground. This "horny wiper" problem is a fairly common one on the 740's. A typical problem is that two bolts that anchor the steering column to the dash structure work loose slightly, causing: A.) A bad ground, hence the various electrical accesseries feeding back into each other, and B.) A looseness in the steering column that you might detect when turning corners, or if it's real bad you may even feel it by yanking the wheel vigorously from side-to-side.

When you said the problem disappeared temporarily by turning the wheel, it makes it sound like that's your problem. Access to the (2) mounting bolts is gained by removing the knee bolster. The factory used shear-head bolts when originally assembled, I'm told as a theft deterant, so in order to replace them they need to be drilled and removed with an easy-out. They should come out pretty easy though, as they probably aren't very tight any more. They can be replaced with regular hex head bolts. Someone advised me to remove and replace one at a time so that the piece over the top of the column doesn't stray from home.