I have a 05 2500 4x4 cummins auto quad cab truck has 315,000km on it runs good but when I'm on the hwy and engine is at 2000rpm when I let off to coast it seems to studder and the rpms jump around almost like a missfire. I've noticed that when I'm at idle as well and when I give a little throttle and let off the engine studders and rpms hold for a few seconds before they drop down.

I have no cat and 4" all the way back exhaust and a Smarty JR. TQ3 TM2.

Any sujestions on what my issues is? injectors sticking or injection pump??

Thanks Regert