My E420 started idling rough a few days back and I'm looking for a little help diagnosing the problem. The Check Engine Light came on so I took the car to Auto Zone for them to run the code. The code (if I remember right) was P0300 which I was told is a random misfire. I started researching some things online, replaced the spark plugs which needed to happen anyway, and took the car back to Auto Zone, they told me the code was now P0306 (misfire on cylinder 6) so I figure at this point it's either a coil for that cylinder, injector, or mass air flow sensor. I noticed some people with similar problems on this forum that had replaced the MAF.

I switched the coils on cylinders 5 and 6 to see if the problem would move to cylinder 5 and the car started running even worse. When I took it Auto Zone this time it showed a misfire on both cylinders 5 and 6 so when I got home I switched them back and it's now running better (like on 7 cylinders better).

Any ideas of what is most likely the problem here?

The car has 136,xxx miles on it