When you say "stall," I presume you mean that the loco is still getting track power, but it stops moving because torque isn't sufficient to overcome the friction at very slow speeds. The root cause is a poor, toy-like gear ratio combined with fairly small motor(s), and there's no easy way to change that. As Allie mentioned, modern-era practice of putting rubber tires on both sides of the same axle doesn't help. Yes you could install ERR or PS3 speed control, but that's tantamount to putting a big band-aid on a fairly serious wound.
Atlas wants to sell trains, and technically your loco will make it around O36. But I'll bet most folks who buy these have O72 curves and switches, or more likely, dream of building an O72 layout. If you're stuck with O36 and determined to run six-axle, scale-sized road diesels through that crossover, I would sell it and get something with Legacy or PS3.
