The XR3000/4000 series are set to regen at 4 hours or 60% soot load (after 80% it will go into limited power mode)which ever comes first, the new XR3100/4100 series are set to regen at 50 hours, and I am guessing the same soot load. It would be a very safe bet that fuel quality will be affecting soot load and when it hits the first limit it is regening, with fuel conditioner or non-bio diesel fuel you may get more hours between regens.