I don't have a pull behind single wheel MC trailer, but have seen many designs while searching the net for trailer designs.
Not much others have not seen but it seems most have some type of suspension, usually simular in design to a swingarm.
I have noticed too, the trailers shown here are attached via a universal joint allowing it rotate, are different in that most other designs the trailer is rigid left/right and leans with the motorcycle and follows it's path.
I believe that being able pivot in that plane is what makes the Uni-Go unstable, just aguess though.
Here's a quite different design, with a high hitch attachment, above the tail light, to were a normal rack would be, with no suspension.
http://eaglematetrailers.com/unique.htm
And this one that attaches to the rear axle of Dual Sport bikes, has what is simular to neck bearings to articulate around corners and follow the bike lean.
I believe this guy is developing a model for street bikes too
http://www.moto-mule.com/
https://www.facebook.com/MotoMule
So it could well be the trailer design, and not the hitch causing the problem
Ken