Rod, interesting that I found your post on a search. I have exactly the same car... model and year, and am experiencing the same problem with my idle when starting cold. I just did my motor mounts and had cleaned the oil breather box and carb while under the hood. The car started exhibiting the problem you describe after completing my work. I do not have an answer for you but was headed toward the temp sensor myself. I will be pulling the intake manifold anyway as I have a crumbling gasket and want to replace it. From what I saw when doing the motor mounts you should be able to get at the sensor without pulling the intake, as long as you get most of the other stuff out of the way.

I hope your post gets responses so we both can solve the problem.
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Chas Mike, former '85 760 Diesel (loved it!); now '89 740GL 230k and counting