Maybe you should double-check all of the linkages and the attachment of the idle adjustment cable. Since the problem went away and hasn't come back my first thought would be that something in the throttle linkage was hung up. You have been around the 240 longer than I have though. Just my .02.

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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