It could be the injectors. If one is not spraying correctly that cylinder will miss, then the fuel from that miss will pass through with out being burned and cause white smoke when heated by the exhaust of the other cylinders. Once it is warm, or when the GPs are on the poorly sprayed fuel will combust because of the heat.

Edit: I hope this makes sense, I am got up too early and am dead tired. :p

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine