Before you go to much further expense and digging into other possibilies, it might be a good idea to double check your valve adjustment. When I first got mine, they were too tight as well. The first adjustment helped but I read a thread, here on this forum, that recommended running it about 100 mi. and doing it again. The second check and adjustment did wonders for smoothing out the running and idle.

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Sam

84 300SD 350K+ miles ( Blue Belle )