Engine running rough

I'd start with a base engine compression test to ensure the base engine itself is not part of then problem. If the cylinders all pass compression (especially number 6), and there is no leak down, then I'd check the ignition system - coils / spark plugs. Even swap the number 6 coil for another to see if the concern follows the swapped components as you move them at a time. Something will show up. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops