Hi There, I have had this exact problem with my King Class. First thing I would do, if you are comfortable enough to do this, would be to remove the tender and loco body and check the soldering - on my example this was all over the place, which may have been a contributary factor in it's poor performance. Additionally, I'd check the contacts from the wheels and clean the pickups/back of wheel where they contact. Again, if you are comfortable enough to, maybe try removing the motor and testing the chassis for tight spots in the motion, and running the motor out of the body to ensure it all runs smoothly. Hope this helps TVR