I've also tried the "traditional' tach fixes on my '83 300D T, but to no avail. Like Flybob, I traced the tach cable to the flywheel. There does appear to be a feature of the cranshaft pulley that creates inductances every time it cycles past the sensor at the end of the tach cable. This allows the RPM's to be counted. My sensor was totally gummed-up with oil and other diesel crud. I cleaned it with WD-40, but still had an intermittment tach. I'm now thinking that over time, the heat of the engine has aged the wires sensing the rotation of the pulley. I'm getting intermittment connection somewhere.

Another strange thing with my tach is that is works when I run the heat in my car whenever "something" kicks on every few minutes to circulate coolant (my guess). I think my problem is mostly electrical do to this phenomena. Any thoughts would be appreciated!