Just one idea left. Have your mechanic let the intake manifold suck in a can of SeaFoam cleaner. If done correctly, it will remove any carbon deposits in the combustion chamber. Lots of smoke initially, but if you have hot spots from deposits, they will cause premature ignition when the engine gets hot.
Be careful if doing this yourself. The vacuum will empty the can in a few seconds if not done properly and you could bend a connecting rod or 2 (new engine). The can must be sipped by the engine vacuum over a 2 or 3 minute period, then shut off for 30 minutes, and at restart it should blow out the carbon. It will also help clean the valve seats, etc.
Klaus
PS On vac for a week.
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I still miss my 164
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