Supermomof4 First, what do you mean by jerking? Is there an audible clunk associated and somewhat random or is it more of a vibration. Can you feel it in the tow vehicle? What does it feel like (pulling you back, sideways, or just a random impact)? Is there any correlation with an event in the road such as a pothole or seam in the cement? I don't think there is such a thing as too light of a trailer so you could probably rule that out. I would start with the ball and make sure that is tight in the hitch? From the info we have so far that sounds like the most probably cause because as you increase speed the wind resistance on the trailer will index the shaft of the ball in the hitch, BUT, if you were to turn or brake you should hear at least 1 "clunk" again as it indexes to a new location in the hole. It probably isn't a tongue weight issue as if you are too light on the tongue it will get worse as you go faster and can get down right scary as you approach 60mph. In my experience they don't tend to smooth out as you go faster. However, if you are too light on the tongue and have a loose ball you would definitely hear something. An out of balance wheel or flat spot will cause a vibration but usually isn't noticeable at lower speeds and unless it was really bad you probably wouldn't notice it in the Tow Vehicle. There could be mechanical damage to a shock or spring but that should remain pretty constant over the speed range and correlate with hitting a pothole or imperfection in the road. Did you find out if you have trailer brakes?