Create a HEMETT and attach to it two Standard Cargo Containers on top of it. The center of mass seems not to have changed. It even gets ridiculous if you drive sharph corners on a hill.

To fix this, please calculate the center of mass if you attach somehing. But store the pivot point of the parent object and don't change it, because the 2nd AttachTo parameter should be related to the same point in space of the parent.