When ihave my wipers on for a while, they start to get really jerky. Motor runs fine, it's just instead of the blades wiping smoothly, they start to grab really bad and then release and grab again, mostly on the return to park. It gets really annoying. When the windshield is squirted with wiper fluid, it goes away, but once it gets cleaned with water again, the wiper blades do it...any ideas??