Hey Rolling-Thunder ... I know this is a really old post, but I have new info. I have the same problem with my wipers, and my problem is definitely in the motor.
They would never park, and they always got stuck midway up the windshield in Interval mode. So, I tore things apart to find out why.
To make a long story short, I took out the motor, opened it, and cleaned it out. Then I reinstalled it. Without the wiper linkage connected, the motor worked fine. I could tell that it was "parking" itself by the way it continued to rotate for a little bit when I turned off the wipers. But when I connected the wiper linkage, the wipers would not park right.
I figured out that the problem is this: There is too much play in my motor. The load of the wipers pulls up on the motor's output shaft. When that happens, the contacts at the bottom of the shaft can't make the proper electrical contact with the plate on the bottom of the shaft. Therefore, the wipers can't park right. I know this is the problem because if I held the shaft down with a stick, the wipers worked fine ... but when I let up, they stopped. Maybe a bushing replacement would fic the problem, but I didn't have time to disassemble the motor completely.
I did, however, have time to get my finger stuck between the plastic cowl and the wiper linkage ... the cut should be healed in a week or so. I hope.
Maybe I should name the car Shylock, since it already got its pound of flesh.
Hope this helps!