Short answer is your engine is "dieseling" or "running on" because of hot spots in several of the combustion chambers. This problem is usually caused by glowing carbon buildup, and is mostly found on the higher compression gas engined cars. Some hard running might be in order, sometimes called an Italian tune-up. The premium fuel and the octane booster you are using will help some during normal operation; however as long as the carbon deposits remain, there will be problems with "run on".

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150