New Santa Fe 4-8-4 Northern N-scale running uneven

[trockne]

I received a new Santa Fe 4-8-4 (#2910) and it runs really smooth in reverse but it has a hitch going forward so it rocks back-and-forth slightly.  Any ideas?  The wheels look like they are quartered properly.

[gatrhumpy]

It may be the 'slop' that is built into the locomotive to handle tight curves (i.e. lateral motion). Not sure how to fix that, per se.