Just got a brand new, green RZR.

I noticed at about 55+ MPH, if I press the accelerator anymore at this speed, it seems to misfire a bit but when I let back off of it, it smoothes out. Is this the governer kicking in on it?

 

sounds like it to me, But I dont have a rzr so I cant be sure. But usually thats what a governer does.

 

Yup, your hitting the rev limiter. If you want to go faster, and who don't, remove the plastic spacer on the primary clutch. It will allow the clutch to close more and the faster you go.