OF 4-4-0 hesitating

[fm_tx]

Hi all,

I have an outside-frame 4-4-0 that has developed a hesitation as it runs.  It acts like something is binding or sticking....about in sync with the revolution of the drivers.  My suspicion was that the valve gear was binding somewhere, but I don't see anything amiss.  I haven't taken it apart to examine the gears...I was hoping to avoid that.  It has a TSU-1000 decoder in the tender.  Any suggestions what I should be looking for?

Thanks,

Paul