I own three Jatos and it sounds to me like your servos may be toast. I had the same problem with my steering servo a couple times and when I took it apart I found the gears were demolished. The throttle servo isn't as prone to damage so that's a bit confusing. I would check the antenna wire and the battery pack and servo wires to make sure you don't have any exposed sections or cuts. If you've got fresh batteries in then you need to start checking wires and the servos themselves. Let us know what you find.