If the intermittent wipers are stopping in the vertical and not stopping in the proper position, then it's 100% to be a worn copper contact inside the motor gearing mechanism. There are 2 sprung copper contacts inside the gearbox section of the wiper motor housing. You'll need to remove the motor and replace it completely, or take it apart and repair it. If you have a drill, some flux and some solder, and are fairly adept at mechanical DIY jobs, then it's quite simple to do. What you'll find is one of the copper plate springs has worn down too far, and you will need to re-shape a new piece and solder it on. When done, put it all back together with either some new rivets of some nuts and bolts. This is an extremely common problem with these Astra G wiper motors.
Stu
March 2013