I have a 4x8 oval and a figure 8 inside.  Black ez track with seperate controllers.  THe 440 engine will only run at high throttle and is jerky(fast,slow, fast)on both tracks.  Checked all track connections.  The 060 pacific flyer runs perfect on either track. Is there a problem with the 440 engine?  What is my next step?

This engine has no standard coupler. The tender has a vertical post that inserts into a slot on the engine.  The tender derails constantly it seems to be due to this unusual coupler.  Any suggestions.  Am I missing a part