Cat 289C wont move / moves slowly

Cat 289c skid steer, 2010. Everything works fine on it except it has serious trouble moving forward and reverse. It will start out fine for the first second and then it slows down to barely a crawl. Same in reverse, throttling up seems to help, which makes me think it is a brake problem?? It will spin left and right fine, full speed, bucket works fine. It did have the code for LH joystick under / over voltage in its memory, no active codes. I dont have cat ET....but the monitor in the cab shows full percentage of the movement of the joystick. Any thoughts? Thanks

Cat 289C wont move / moves slowly