ryan,
i have exactly the same problem sometimes when the starter gets hot. cycling the key has gotten it to work again.
rickjordan also had the same phenomenon. look up his post on the subject. he solved it by replacing the starter. i still have my old starter, but am just waiting for the hot weather to have a no start situation happen again..

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