My '84 300D had the same symptoms last week. Heat was great at idle however with increased speed it cooled off. Replaced monovalve and now it works great!

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'96 C220 138,000mi, '95 E300D 239,000 mi., '87 300TD 214,000mi '88 6.2 Turbo Diesel Chevy Conversion Van 253,000 mi.