D6T joystick problem

I have a 2011 D6T dozer that dosent lower the blade like it should. In the past the blade had three positions; when you pushed it ahead a little it would lower slowly then if you push it ahead even further it would drop really fast, and clear ahead it would go to float. Now when ever I run it and push it ahead 3/4 of the way it drops really slow, and when ever I push it all the way ahead it causes the cylinders to chatter loudly and still lowers slow. I wonder if there is valve some where...

D6T joystick problem

D6T joystick problem