And yes, when an engine becomes troublesome like this, I clean the wheels AND the axle point cones.
I also mentioned possibility of lost contact in a 3rd area: between the trucks metal finger/nub and the springy strip in the chassis. The trucks have fore/aft play (and lateral play) in their mounts in the frame. When changing direction they will shift slightly in the new direction of travel until they shift enough to start moving the entire model,
