is there a error code on dash or in error log.
you can remove electric magnetic brake and put on floor where operator feet are while still connected to wiring harness. then jack unit of ground and drive unit to see if at full speed. if not the encoder for drive motor could be u/s. by the way if encoder is fitted wrong by pushing on the out side it will properly no good. it could be the traction controller playing up. swap the hydraulic controller with traction.