Mine doesn't do this. When I've worked on my dad's brass HO engines, the hopping tends to be because the screws on the gearbox have loosened. Maybe check if that is the issue. Taking the engine apart is not too hard, at least when I did it in mine (just 4 screws to remove). The only fragile part were the wires connected to the hands rails as the antenna.
Another possible issue is if the wheels/rods are not quartered which is strain on the wheel turns that could cause the hop with the engine trying to roll the wheels.
