I have an '03 400 ACT, with what feels like loose handlebars. You feel it when the suspension flexes over bumps. I went underneath and checked the tie rods and steering shaft, I couldn't feel any perceptable play in the linkages. There is a collar just underneath the plastic, could this be loose, causing this?

 

I noticed this also, the collar under the plastic on mine is tight but it has slack in it. The collar has plastic bushings that do not fit tight on the steering shaft. It appears to be a poor design.

 

Mine was doing the same thing and I tracked it to the steering shaft. Where it connects to the rods it was loose. The 2 bolts had backed off. There are cotter pins keeping them from coming completely off but they were really loose.