Following this thread! Despite the O31 rating, those are sharp curves for this loco's longish wheelbase (compare to a 773 for example.) Rubber tires don't allow the wheels to skid and it has one-piece side rods, so the axles can't move very far laterally either. So as soon as you enter a curve there's a dramatic increase in friction and corresponding load on the motor.
I'm guessing your issue is a circuit board or speed sensor. That being said, I have a Legacy Atlantic that's contemporary with the 6-11150. Mine exhibits a stuttering or shuddering at certain speed steps which I couldn't resolve. I think mine is due to improper mesh between the worm wheel and the worm gear on the motor shaft. Unfortunately on both the Atlantic and the F-12, Lionel cut some corners by pressing the worm gear onto the motor shaft, instead of using a separate gearbox to ensure that proper mesh is maintained at all times. Would have gladly paid $100-$200 more for the better design.