I noticed in my Charger model, and Chargers my friend owns that the gear case is too wide in one area allowing one side of one of the idler gears to come out of its bearing (hole) in one direction only. It still works but it is not optimal. I did not experience any surging (with ESU LokSound decoder installed). Becuase I have seen 3 models with the same problem I assume it is a widespread issue. And it is not the gear that is problematic - the sides of the gear case are just spread slightly too wide.
It's been a while since I looked at this loco. I recall an easy way to check for the problem is to hold the model upside down and using a finger try to move the wheels on each side of the truck while observing the corresponding idler gear. Normally the idler gear is centered in its slot of the bottom truck cover. But the affected gear will skew to the side of the slot when the wheels are forced to turn.
I totally forgot about this until Mark mentioned what Kato told him.
