FD-Man

Have a 2004 Chevy Silverado classic 2500 with a 6.0 motor… Recently rebuilt, getting a flashing engine check light throwing code P0300 which is a misfire only does it around 80 miles an hour when you back off the throttle. Like I said motor was just rebuilt new plugs new wires and new coils, throttlebody is also new, ran a full tank of seafoam through the motor to clean the injectors, gas cap is secure in tight, Anybody else have any ideas on how to figure out why it’s miss firing and how to figure out what cylinder or cylinders it may be… We don’t notice any loss of power at all or shattering in the motor…