I hope that you had a Merry Christmas H!

The spark plugs sometimes are tough to screw in by hand, and if they are, the plugs need to be cleaned with a wire brush, and the threads in the cylinder head could use a thread chaser. Copper antiseize is a wise investment,

Never use a wrench to start a plug, always use a plug starter or your hand.

H, if the plug is leaking, your idle will roughen. to verify if it is leaking, all you need is a tube.use a vinyl or rubber tube. Clean one end and stick it into your ear, then put the other end around the plug, This works very well for finding any air leaks (exhaust etc.) I would imagine if the leak was enough to affect the idle, then you would be able to feel it with your fingers, and hear it with your ear.

You should also check your intake gasket and other hoses for vacuum leaks.
Starter fluid works well for this. A leak will roughen the idle.

My first thought was the O2 sensor.

Good Luck H !!!!