2005 e53 3.0 d - sluggish start - starter or IVM?

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Hi

I've got a problem where the starter motor does not spin the engine as it should. It always starts but you always think it is not going to as engine is span very slowly. I've read elsewhere that it is usualy starter motor or IVM (battery is defo OK)

Is there a way to be sure before I comit myself for second hand starter? I've messure the Amp on crank taken by starter and it is around 400 A. That sounds a lot. The starter is made by DENSO pn 7788680.

thanks