Chevy 1998 z71 5.7 eng. will (sometime) get 304 or 305 or 300 code, only when towing a boat going up small incline.
When going up small incline pulling boat traveling 65 to 75 Mph, truck will get
shutter, service eng lite comes on and give misfire code. Shutter goes away
and engine smooths out after few seconds. Been doing this since 130,000
miles. Complete tune ups, oxygen sensors, catalytic converter, intake
gasket,spider injectors and sseafoam in eng and gas also Transmission
Rebuilt.