I disconnected the 12v feed to each coil. I got the expected response; a noticeable miss and check engine light with the corresponding code. No cylinder indicates and obvious problem. But I still have the roughness at idle.

While poking around checking for a possible vacuum leak I noticed a faint hissing sound from the right rear of the engine. I can't nail down exactly where it is coming from but I will remove the intake tube and inspect the area. The EGR valve is in that area.