why does my uni-wiper keep jumping out of the groove? 87 300E
 

Whenever I turn my wipers on, after a few swipes, the wiper jumps out of and on top of the grooves that it is supposed to swipe into (black pieces that run up the side of the length of the windshield). It then proceeds to be loud and obnoxious and smack these black pieces with every swipe. It also leaves a big unwiped space in the center of the windsheld when it does not go into these grooves. What's going on here?

I might need a new wiper motor, because the speed of the wiper fluctuates depending on how fast the engine is revving. Could this somehow be the culprit?

Thank you for your time.