I have the same problem with my 300DT-I am going to do a compression test and get my injectors cleaned and balanced-if you do a search, you'll see that those are 2 common causes(along with the much harder to fix stretched timing chain problem). Does your car fly down the interstate smooth as silk and then shake and smoke like it's possesed on the offramp? It must be mine's long lost big brother!

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1982 300DT 190K (Diesel Purge + synthetic oil=smoothness at last!!!)
2004 Ford E-350 6.0 L PSD 227K
2006 Dodge Ram 3500 SRW HO Cummins 4X4 48RE 42K (brute force tow vehicle)
2005 Scion xB wife's rolling pop can
1993 GMC Sierra C3500 6.2 142K