Hi Bill,
Check the spline drive adaptor that the wiper arm attaches to, these have a habit of wearing or running slack(held on with a grub screw). The other bit to check is at the back of the dash, the wheel boxes clamp onto drive rack and the clamp bolts slacken with the vibration of the wipers working. A wee bit of wear or slack in these cause the sweep of the wiper arm to vary by a huge amount especially on a wet window.
cheers,
Stevie.
