Put a strait edge on the track. It sounds like a small dip or rise in the road. The transition over it is steeper one way than the other.
I.e. it is hanging off a cliff stiffly on the drivers. (while the pilot wheel drop a bit and stay railed) The right rail may have a dip, or the left a slight rise, or the combo.
You could also try to see if it teeters (front to rear) there at all by slightly pushing down on the nose & cab.
