Hi Irzigen ,
I am not getting the last driver you have this could be in conflict with the pelvis/throrax + thorax/skull driver.
The LateralBendDriver sets the linear measure y direction to zero, but this is the y direction of the global ref system, since the "Ref" property in the AnyKinLinear object has not been set meaning it is "-1" so global y direction in this case.
As i see the markers and the code, you a four markers on the sternum this should be enough to drive thorax since the other two drivers will make the spine stiff, then you do not need the last driver it will only cause trouble,
Sorry if i have missed what the issue is 
Best regards
Søren