Car starts up fine and the idle is smooth for the first 2-3 seconds but then abruptly transitions to a very rough idle. Checking the exhaust flow, it is dead smooth for those first 2-3 seconds but then starts to pulse strongly. Driving the car around the neighborhood, it continues to run rough.
CEL is illuminated and the code is P0301 (cylinder #1 misfire).
So far, spark plugs, wires and ignition coil have been replaced with no change.
Anyone have any thoughts on the most likely cause?
Your car has a cylinder #1 misfire generally caused by a failing spark plug or spark plug wire. Both should be replaced every 30,000 miles on this vehicle.
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Thank you for your prompt reply. 
The spark plugs, wires, and ignition coil have all been replaced with no change in performance.
(I’ve updated the post accordingly)
If you have any other ideas, it would be appreciated!
Try swapping the fuel injector from cylinder 1 with an injector from another cylinder, to see if the miss moves with the injector.
Tester