I went down the road of trying multiple CCU units on my 87 300D. I ended up following the instructions by forum member James Dean, and was able to replace the capacitors inside the unit. Mine went from flaky operation to perfect operation. I also re-soldered all of the points on the bottom of the circuit board. There is a good chance that flaky-operation could link back to cold solder joints and/or bad capacitors. The boards on these are pretty simple as far as electronics go. I don't have time to look it up right now but if you search CCU solder or something like that I bet you will run across his posts.

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97 S500 (90,000 miles) - wife's car
87 300D (298,000 miles and a replacement #14 head)
94 Suburban K2500 - need something to haul firewood
83 300SD (343,000 miles) (sold)