I would test the cold start valve. Without extra fuel being injected, most gas engines are very hard to start when ambient temps are low.

Also test the kickdown switch.

As far as the spark plug...it sounds like your mechanic already knows how to handle it. Only other option is to replace the head.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles