Going all the way back to when they were released, I've always read that these locos weren't good runners. The motor itself is nothing special. However, removing and reinstalling a worm gear isn't trivial. (There was a recent thread on this very subject.) The key is finding a worm gear that has the correct pitch and tooth depth to match the worm wheel on the axle. Replacing the worm wheel is a whole other project!!
Could it be that some of your driving wheels are not quartered properly? Maybe the quartering jig used by the factory had an error, which is responsible for the sub-optimal performance? Again, checking and changing the quartering won't be easy. Maybe you'll be the first to solve the mystery of poor performance with these locos.
