Also try cleaning the engines wheels, as well as your track.
Springfield PA
If it runs smoothly as slow speed, that's probably not the problem. If the mechanism was stiff, it would bind up more at low speed. Before trying anything else, reset the NCE decoder by setting CV30 to 2. With the dual decoder setup of Blueline, this might not be as simple as putting the loco on the program track and doing it. It's possible there are several things wrong, the decoder could be programmed to use other than 28/128 speed steps, it could be set up to use a speed table that has bad values, or it could have odd values in the start/mid/top speed CVs. Reseting it will clear all of this - when in dount, try a reset first. After resetting the motor decoder will be on address 03, before programmign it back to the desired address, give it a test run.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Don't worry, your English is far better than my Spanish would be.
Your problem seems rather odd. If I understand what you mean, your loco runs very well at a slow speed, and runs very well at top speed, but does not run in the middle speed ranges? Are you using DC, or have you installed a decoder to run with DCC? If you are using DCC, what decoder did you install? Usually if the mechanism is 'tight' and needs some time to run in, there will be jerks and hesistations at low speed. What exactly is it doing when you run in the middle speeds? Stalling completely? Does the sound cut in and out?
Hopefully we can figure this out for you so you don't have to ship it back for repairs, which is likely expensive (the shipping - since it's brand new there's a warranty on the repairs).