x475 forward foot peddle problem

When I have it at full throttle and the forward foot peddle all the way down I don't have the speed I use to. Also when I take my foot off the peddle it now comes to an abrupt skidding stop. We took it in and had John Deere fix it once before for the same thing but didn't find out what they did. I was hoping it would be something simple I could adjust. Any help would be greatly appreciated.