Curious about this

Why do some TnA Engines seem to function perfectly then suddenly, They preform the wrong line or function when they run over a certain track? Like for example my Gordon every time he hits a 5 mark, he does like the function of 4 marks or 1 mark? What the heck? I've tried jiggling the tab reader but its no good unless i run the track on him when he is upside down, How do I fix this and why does it happen?