George, few years ago I did a complete tear-down and tuneup on the engine and all the cars. The articles were published in the N-Scale Magazine few years back (I can look up the issue when I get home).
If the binding happens once per driver revolution then it is most likely the siderods/valve-gear related. Unfortunately, the best way to troubleshoot this would be to have the running gear free-rolling (means removing the motor). Or, if you can get the model to stall when it binds then you can take tweezers and carefully try to gingerly move every piece of the running gear. The one that binds will likely be jammed (or more difficult to move than the others), while all the other pieces should have some play in them.
