did you try jumping the 2 wires on the black box on passenger side fender?
(not sure if an SD has them??)

i've had this problem for years, mostly when it's hot out...or after a long highway run. cycling the key has sofar never failed to engage the starter.
another member (rick jordan) replaced the starter and it cured it.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP