Bachmann GWR 4575 Tank

I don't know if I'm being too critical about my locos or not, Maybe I'm too much of a perfectionist.

Last week I purchased a Bachmann 4575 Tank which was kept in A display cabinet at the Dealers, I Don't really think that it is much of a problem as other Locos which I've purchased were also kept in such a cabinet. She was also oiled before the usual running in, as the oils dried out in the cabinet.

Today I noticed very erratic/jerky running at high/scale speed which is apparent in both directions of running when pulling one Coach or Wagon. My 45xx (Which is currently undergoing a large overhaul, Motor, repairing the lining...) also suffered from the same problem, so is it possible that the problem is directly attributed to the Chassis?

The 4575/45xx were known for their high speed running capabilities in both directions so it would be a shame to keep it running at a slow speed.

Any ideas?

- Jeorge