Intermittent Misfire

If the cylinder missfires, whether rich or lean, the ignition system can be perfect, yet the light can show a skip.
Lay the plug wire on any grounded surface, and start the engine. Now go observe and listen to, the snapping going on at the end of the wire. My guess is that it will have a nice steady rhythm to it.

Since you earlier mentioned that a valve adjustment temporarily made the engine run better (stopped the popping) My guess is that you have a problem with the valves in that cylinder.
My guess would be it is with an exhaust valve. And
My guess would be that it has insufficient closed valve spring pressure. When this happens,; on the intake stroke, the piston is able to pull a little bit of exhaust back into the cylinder. This reduces the pull on the plenum. So that cylinder at times, may not have the correct AFR at idle to support combustion. The flame goes out. The partially unburned mixture enters the exhaust. It may be fired up by another cylinder coming up on the exhaust stroke and ... POP, there it is.
So, pull the valve cover and look; there is no other good test to prove this. Put the piston of the cylinder in question, near TDC compression. Check for a broken spring, or a spring that is easy to spin (compared to several others). Or lean on it a little, to make sure it doesn't want to fall into the cylinder; just don't drive it into the piston-top.