My dad has a brand new Gary Fisher Pirhana with a rigid and suspension corrected fork from Surley. When he's going from 16-24 miles an hour and takes his hands off the handlebar, the wheel and handle bar vibrate left and right (as if it's steering left and right 1-2 times a second) of their own accord.
New tires effected the magnitude, smaller tires resonating to a degree of about 10-15, and thicker tires resonating at avout 20.
We placed weights inside of the handlebars, but that only dampened it.
Sometime today or tomorrow we are going to switch my wheel and his wheel and see if it still happens.
I'm beginning to suspect that it's just this frame/wheel/fork/handlebar combination that happens to resonate badly, but if anyone can shed light on how to fix this it would be greatly appreciated.