This happens on the first wipe (which might be as its coming out of the 'park position) then sometimes for the next 2, 3 pr even 10 wipes... The number changes at random - though flicking onto the fastest setting sometimes 'throws' it to the proper sweep.
The sweep is not controlled electrically - it is mechanical ( apart from a certain condition where the motor does go into reverse in order to reverse the mechanical control - more on that later ).
If you look at the wiper linkage, just above the motor spindle is a short arm that has, what looks like a plastic hat on it.
You can see the bit I'm referring to in the photo here -
It's the black hat shaped cover with the splodge of green paint on it.
It looks like a simple click on bolt cover. Nothing clever at all. Doesn't it?
However, it is a clever little device that allows full rotation one way but only limited rotation the other. With the wiper arm out of the car, try it and you'll see how it works.
Unfortunately, they seem to gum up a bit and don't work properly. This is why you can force the wipers into full sweep by pulling them. It's also why it appears to be a completely random amount of time until they rectify themselves each use. They just have to overcome the resistance.
I did improve mine considerably by simply cleaning the underside of the device. Unfortunately, I couldn't investigate further as I had to get the wipers back in the car.
There has to be a way of opening the device, cleaning it and repacking with fresh grease. If you can find out how to service the device, I'd appreciate knowing!
The most infuriating thing is that 95% of the last sweeps (when going back into park) are also the 5/6 of a proper wipe!!
That's how it should be. The motor does one final sweep ( in reverse ) in order to relatch the limited sweep device.
Hope that helps.