I've been having this problem for quite some time now, the tranny in my '85 300D likes to almost skip 3rd gear on anything other than hard acceleration. It likes to just spend a split second in 3rd before going to 4th. I've got the bowden cable adjusted to spec but but it still does it. The only way really to keep it in 3rd is to put it in "S" and then shifting it to D when starting out.
This problem's got me worried.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012