What kind of riding were you doing before it wouldn't rev out? What jetting.. yeah yeah I'm always blabbin on about jetting.. My 2010 would load up after about 3 minutes of singletrack using stock jetting and stock plug. Just as you said it then wouldn't rev out freely until after it cleared it lungs and then it would hook in again until I loaded it up in the low revs and same same all over. NGK plugs seem to work better than the stock Denso (I use BR8ES cause they're cheaper and work just as well). I also swapped out my cable mount choke because it kept hanging up on vines etc in tight trail.
So what I'm saying is all the replied above give you something to consider, and I've given you a little more
Lucky bugger with your 2012 model! Hope you get it sorted.