Wipers acting up

The wipers on my 99 F250 SD are acting up. I turned them on and they swiped left then started back and stopped in the straight up position. Now whether the switch is on or off the wipers bump an inch or so and stop. Then after a couple of minutes they bump an inch or so in the other direction and stop again. Anybody ever run into this before?

Could be a short in the multi-function switch (turn signal arm). To get an idea if it is, with the wiper on, give the arm a wiggle. If the wiper works some from that wiggle, then it is that switch.
It could be the motor. I have found that if it is, giving it a tap and getting it to then work some from the tap is a good indicator.
It could be the intermittent switch/timer. Unfortunately I don't know a way to check that.
And it could also be a short in a wire somewhere. That involves checking each one for conductivity.