If the blades judder accross the windscreen it is because they are not set correctly - to check this,turn the wipers on and then turn the ignition off mid-sweep so that they sit halfway up the screen.
Lift the wiper off the windscreen and put it back again almost touching the screen - the blade should be at 90 degrees to the windscreen.
If it needs altering - get an adjustable spanner and close it on to the wiper arm close to where the wiper attaches and twist the arm round so that the blade is at 90 degrees to the screen.
Also make sure you use a screenwash additive and not just water.