Jim,
Here are some drawings I made to understand what is going on inside that little gem: (the other guys here are probably tired of seeing them)
You can pretty much see what is going on. What might not be so apperant is that when the wiper motor is in the "Park" position, the plunger switch mounted to the motor grounds out the normally powered side of the motor's "slow"winding through the dash board toggle switch (both sides now grounded).... in effect creating an electric "brake" which haults the wipers. If you have it wired so that the normally powered side of the slow winding does not get grounded, (when in Park) the motor has a tendancy to skip over the plunger switch cam and continue for another cycle. The higher voltage you talked about only makes it more likely to skip.
So, #1 check your wiring. If it is correct, then start looking for a reason that the windings are not getting grounded when in park.
Steve