Originally Posted by kirklandkie
I was thinking, isn't there some function that alters wiper speed depending on vehicle speed? could this possibly be the culprit?
No I don't think so. Sounds like a bad GEM (I've heard about several go bad) or something in the wiper motor/controller. I would try to rule that out if possible, I think those are a few hundred dollars. I guess it could be the multi-switch possibly? I had a problem like that on a GM once, the circuit board on the motor was cracked.
Also, I remember something about water intrusion concerning the GEM on f150 and expeditions. Poke around the GEM and see if you notice any corrosion. Also, check if the windshield has ever been replaced. A poorly installed molding can cause water intrusion.