I had this problem with two MTH Imperial steam locomotives. I too thought I was not getting the drawbar plug seated securely & it would slightly disconnect about once every 10 times around the layout on only one large oval section. I found a section of track that had a short, tiny up & down area and after going over this section those many times it would start to work it's way apart, but never over switches. It was hard to find. After I replaced a section of that track the problem went away. I hope this gives you another way to look at the problem.
D.
