Haven't seen exactly my problem posted so thought someone may be smarter than me and narrow it down. My 82 240 run very strong (sometimes feels like a non-turbo 300D), uses very little oil and normally hardly smokes. Every few days it will lose about half power or more and smoke like crazy. I almost have to put off on the side of the road. This may last for 15 or 20 miles and then it "breaks out" of this problem and runs perfectly. I have ran a gallon of Startron tank cleaner, changed the pre and main fuel filter several times and it seems there is not an excessive amount of crud that shakes out of the filters. This intermittent problem is so annoying. any ideas?

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85 300D 107K present
82 240D 190K present- next-to-slowest car I ever owned
81 240D 1995-2000- slowest car I ever owned
79 350 SE Euro 1996-2001- fastest car I ever owned