Tach problem and rough idle is very likely caused by the over voltage protection relay that is located just behind the battery ,a silver relay with a red top and a 10amp fuse at the top.If it is intermittent it is more likely the relay failin than the fuse but it would not hurt to replace the fuse and hope for the best . Easy to do and cheap,however the relay is not a real hi dollar part either. Don

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97 E300D 157000 miles
87 300TD ?141k? miles