Based upon the interaction with the tach and the EDS subsystem which controls idle speed, this sounds like the part to change. I seem to recall that it is behind the battery and is fairly easy to change. Correct? Final question: what are the cheapo brands? This site only offers one brand and the brand name seems to be URO-ish. Dealer item?

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87 300D 265Kmi
Factory rebuilt crate 603.96x engine at 200K