Won't start

  mercedes/e-class/2010-2016

2010 Mercedes e200cdi sport (w212)
Ignition comes on but car won't turn over. After a few attempts or sometimes after nearly an hour of trying it starts perfectly. Mite do it the next several times I try to start it or mite not do it again for a week or two. Runs perfect when started. Any help appreciated. Thanks

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