Replace the spark plugs. If the check engine light continues replace the coils on 6 & 7 or swap the coils to other cylinders to see if the misfire follows the coil.

Fuel of different octanes have different ignition characteristics. My guess is that 6 & 7 have weak/marginal ignition that just squeak by on the burn characteristic of premium/higher octane. Run the proper octane from a top-tier brand in the car.

No I don't think it is injector related although 14,000 miles of regular has not been keeping them as clean as premium.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer