Hey Beachhead,
I purchased a new (old stock) Green 3243 set from Sidetrack in 2014. I had the same issue. It ran on straight track but was jerky on curves. It had a brand new battery and no wear on the mechanism.
I contacted MTH and found out there was a program update that was not on the web site. MTH sent me two files.
I live near Stockyard Express in Oberlin, Ohio and took the engine to Bryan who is a wiz at MTH repairs. He hooked up his computer and upgraded the program installed a BCR and the engine runs perfectly. I am not sure which of the two files is in my green 3243. I highly recommend Stockyard for fixing any MTH issues.
I have never uploaded anything but photos and videos but I will try to upload the MTH programs. At least you can see the file names and get the files direct from MTH.
Hopes this helps.
Jim Z