Not knowing whether it is a 4cyl or V-6, I'm assu8ming it is a V-6.
1. Check for a potential exhaust flow restriction of the front catalytic converter. Remove the Oxygen (O2) sensor and see if the misfire code and running condition are still present.
2. If the running condition is still present with the upstream O2 sensor of the front head removed, perform a cylinder contribution test of the front three cylinders and at least one cylinder of the rear head to verify all cylinders are showing an equal RPM drop.
To do this, monitor the RPM change of the engine when unplugging the individual COP unit connectors to kill the corresponding cylinder.
3. If cylinder #4 and/or another cylinder of the front head shows an insufficient change in RPM, inspect the camshaft lobe and lifter movement of the related cylinder and service the valve train or head as needed.