I have a 400 pound trailer (Hobie AI) that hops if I tow it with one car but not on another car. The only difference is the hieght of the hitch - but this doesnt make a lot of sense to me. On the car that it hops on, I can add some weight on the tounge and that helps a little. I just replaced the tires and the old ones had a real strange wear pattern - like no tread in one area but rotate the tire 180 and there is good tread.. Maybe I also have a little alignment problem with the axle on the frame but havent checked (even though I have owned this trailer for about 10 years now.. one day..)
Also, if its a single axle trailer you have to pay more attention to the weight distribution and balance. Single axle - single pivot point. If you either dont get the boat forward on the trailer or put too much load weight to the rear, you can also have an unstable trailer. I know on the 26S, 150 pounds tongue weight is too low but mine tows well when the weght starts to get close to 200 pounds.
woops.. I just read your original post - no boat on the trailer..