Runaway simulation in gazebo 9?

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Hello list,

I have a simple model, consisting of 4 boxes/cylinders and 3 revolute joints, very basic. It is controlled using the joint trajectory controller with a simple python script. It is not connected to ground, it effectively is a box that can push itself on the ground with the rest of the model.

Here a short video, exported from gazebo that show's it in action.

And here the complete project.

But after 3 second the model starts to "oscillate" violently. The joints suddenly don't seem to be revolute anymore. This smells like a bug.

Using gazebo-9 ros-melodic from source on debian testing.

Edit: Somtimes gzclient crashes with:

gzclient: /build/ogre-1.9-1.9.0+dfsg1/OgreMain/include/OgreAxisAlignedBox.h:252: void Ogre::AxisAlignedBox::setExtents(const Ogre::Vector3&, const Ogre::Vector3&): Assertion `(min.x <= max.x && min.y <= max.y && min.z <= max.z) && "The minimum corner of the box must be less than or equal to maximum corner"' failed.
Aborted

Is there anything I can do? Thanks in advance, Sietse