If starting in neutral works, problem is probably neutral safety switch. Our '95 e320 has this problem, sometimes starts in park, always in neutral. Seems to start in park in hot weather more.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro