First situation did sound like a vacuum leak. The rough idle and wondering rpms.

I think the original plugs should be re-installed to address the 'still runs like shit after the idle issue was resolved' thing.
The coils saturate and discharge at a certain controlled rate and no plugs but the direct replacement motorcraft/autolite seem to get the resistance value right. You can also check the spark gap. Plugs sometimes have more than one a pplication so they aren't always gapped for a certain motor.

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08 4.0 Sport 4x4
Stock