If you take off the cover that is next to the motor and very carefully clean the large circular copper clad electrical contact, you may find that the motor works better. I clean them with a q-tip and alcohol.
Don't mean to Hijack, but I've had a similar question about my '76 for awhile. My "park" position is right in the middle of the windshield. Is is a prettye easy adjustment to get them to park at the bottom?
You might check the ground to the chassis from your windshield. I just repainted and tested my wipers before I covered it all up with the defrost vents. Wipers worked fine but it wouldn't return to park. I ran a ground wire from the motor to the chassis along the wiring harness and it worked fine. Must have gotten enough paint on those hinges!
I had to re-clock the part that attaches the motor to the linkage. It was really tough to get that nut off. The motor was attached and powered. I watched where the motor stopped when turned off. This is also how I confirmed that it was stopping in the same spot regardless of where it was in the cycle.
I reattached the part that attaches the motor to the linkage, with the wipers in the park position.
Use locktite here. Shortly after having it all fixed up the nut spun loose on me during a bad afternoon of sleet and snow. My friend had to stick his hand outside and around and manually work the wipers while I drove. For and hour.
Oh, while you are in there (I am talking about accessing the linkages by removing the defroster vents) take apart and clean the mechanisms and ube with lithium grease. It made a world of difference for me.