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I can offer some advice on how to debug your simulation, but it'll be difficult to give to a solution since I don't have reproducible steps.

Check that there are actually no controller running.
Check the inertia, collision, and joint values on your model. There are visualization tools built into Gazebo to help identify problem. For example, a link's interia visualization (a transparent purple box) should roughly match the size of the link.
You could pin the robot to the world frame, rather than turning gravity off. Zero gravity is not very realistic, unless you are planning to operate in space.