I made a quick video of stills showing my problem. When motion blur is enabled, it will randomly grab objects and place them floating above everything else. This video is me just clicking render to picture viewer several times. Nothing changed between renders, just the results. The Cloners I'm using are set to animate, so in other words, this project is an endless road. There is a group with one section of road, two street lights, a shoulder and the row of barriers. The car itself is only animated with a subtle vibrate tag and there is a camera move.
Notice that the barrier is never blurred, but when one is placed randomly, it is indeed blurred. Not sure if the road surface is being blurred properly. Hard to tell with that material.
If I turn off the motion blur options (I'm using Camera and Object), it will render perfect, minus not having any blur effect.
https://vimeo.com/282547207