Before you replace spark plugs, throw new parts at the problem, I suggest more troubleshooting. I suggest cleaning the MAF with MAF cleaner. The MAF function is to regulate air/fuel mixture which could cause hard starting, rough idle and even loss of power. Before replacing spark plugs I would remove one wire at a time and see if each one makes the idle worse. That will locate the cylinder that is not at 100% if that is the case. It still might not be a plug. It could be an injector, a coil or the wire itself. It could also be oil in the plug hole.

__________________
X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas