The Athearn DCC-ready locomotives are hit-or-miss honestly. The motor is usually high-draw and the drivetrain is questionable. I have an old blue-box Athearn that runs with a LM-1 drawing 700mAh at full load motor current. But I spent some time tuning the motor and the drivetrain to get it to run well....otherwise it would've probably been over the limit.
I have another Athearn DCC-ready locomotive that I placed several different Athearn motors into it in an attempt to get it to run better. In the end I finally trashed the Athearn motors and placed a Kato motor in there. Now it is one of my smoothest runners.
Here's what I would do on yours....First, put it on stands and run it for a long time in both directions on DC. That will help break in the gears. Better yet, take the trucks apart and fine-tune the gears with a small file. Get rid of any burs and rough edges. Then put some pumice or toothpaste in the gear tower and run it for a bit to smooth everything up. Clean it all out and put some quality lube in there. You can try messing with the motor, but honestly it's probably best to just swap it out with a Kato or similar. The older Athearn motor will never be as smooth or low-draw as the higher end motors and takes up way more space.
For the record, I have done the above with all my Athearn blue box and "DCC-ready" locos. They have all had the gears reworked and tuned, and all (save the one) have had the motors replaced with Kato. As such they are some of the nicest running locomotives I have now. Better than Genesis, and pretty much on par with Atlas.
-Kevin
PS - one last thing.....check for anything binding on the drivetrain. The module may be pushing some wires down on the driveshaft which is causing it to bind.
