Ford Repair: 2000 Taurus miss, dpfe, taurus 3

Question
We think we know what it is but need a second opinion.  My friend brought over a 2000 Taurus 3.0L that he found had thrown the po401 code.  The owner of the car also said it was missing.  We took it out and it will buck/shudder/miss slighlty between 1400 and 1600 rpm or between 29 and 33 mph if held at a steady speed.  We checked to see if it did this in all the gear ranges, to perhaps id a transmission problem, but couldn't make a determination.  He did find a split vacumn line going to the dpfe on a previous inspection, which he repaired and cleared the code.  When he brought it over tonight the light was still off, but after driving it for a few minutes at the speeds stated above, it came back on.  We suspect it is the dpfe, but want to make sure that the miss can be caused by this part being bad.  The car belongs to an older lady and we are just trying to help her out, so we don't want to just throw parts at it.

Answer
dpfe will not cause a miss-unplug egr valve and see if problem goes away-light will come back on though-egr is not as common as use to be. always can be plugs or wires-cam synchronizer also can cause this especially if you hear a chirp or squeak at times.