If you have locomotives that have constant stalling issues, despite having clean wheels, gears and axle contact plates, then the culprit is the long contact strips that carry power from the axles to the chassis. With the trucks removed from the body, inspect the contact strips.
Make sure they barely stick out at a very slight angle to ensure the truck contact tab touches the contact strip at all times. The contact strip should NOT hang down at a steep angle, nor lie flat inside the shell.
To fix this, remove the shell and carefully bend the contact strip back to a virtually-flat angle, and after placing back into the loco chassis, using a small flathead screwdriver or tweezers, bend the contact strip very slightly to the desired angle. It should only stick out of the shell at the angle shown.
