My 87 300D had an electric oil pressure gauge. I had problems with it too. It would drop to zero sometimes or not function at all or it would work erratically. Replacing the sender did nothing. A good whack to the top of the gauge pod would get it to work normally for a while. My guess you have something faulty with the gauge itself.
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DJ
84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012
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