My 85 300D is experiencing the same problems. Most of the time it starts but every tenth start or so it does the same thing, the solanoid clicks and the lights dim like its getting current but the starter won't turn over. It will start, however, if I jump it from another car for about a minute or two. I started a thread on this about a week or two ago but I can't seem to find it. It was recomended to me that I should clean the engine grounding strap but that is connected where the engine and transmission bell housing meet. Needless to say its torqued down really well and I can't get it to come loose. A friend of mine suggested that I ground the engine out somewhere else so I'm off to try that and if that doesn't work off comes the starter. By the way, I went to O'rielly's and the battery, which is three years old, tested out at one hundred percent and the alternator is putting out thirteen and a half volts at 700 rpm like it should. I think I've pretty much exhausted my options unless someone else has any ideas. Any thoughts on the subject would be immensly appreciated.
Thanks

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo