I have a Skyjack SJ7127 RT that works great other then when I push the joystick all the way forward or all the way backward it drives a lot slower then if I just push it forward a little bit or backward a little bit. If I push it forward a little bit the high/ low throttle and high/low speeds seem to work good. The flat mode and the hill mode also seem to work good as well. It is only when I get about half way forward or backward is where it slows down. Tried replacing the old joystick with a new joystick and that didn't make a difference. What could be my problem? Thanks.