I have the same issue. I have looked it up via google. Here's what I discovered. It could be the Fleetguard filter being too restrictive. We went from the 9000(or 9100) to the 9080...and it still happens. There is yet another upgraded filter that replaces the 9080. I have read that you may need to have your static timing checked by Cummins which would reduce soot production. Also, a service to the sensors and control modules for your EGR needs done if you haven't already done one(there is a youtube video on how to do it on a Cummins 871).
