Strange wiper problem

I've got a strange problem with my windshield wipers. I swear they are haunted. When my wipers are on, they change thier stroke on thier own. I can have my wipers working normally, with a proper stroke, then on thier own they will start to work on a long stroke pushing the wipers on to either side of the windshield frame. They will work like that for a wile, then reduce thier stroke to less then normal. The speed never changes, just the stroke.

Has anybody experienced this? Is it my motor?

 

Does it over-rotate beyond the rubber gasket?
If so, there's been some discussion on that before and a solution to fix, which btw worked. I'll see if I can find it & post it....
Edit:
see if this helps.

http://www.earlycj5.com/forums/showthread.php?t=5513&highlight=Windshield+wiper