I'd run 205/60-15, and have the airbag contact ring & related parts in the steering wheel replaced. I had the EXACT same symptoms, and this was my fix

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"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo