P0300 code

I have been having off and on acceleration issues for weeks. More of a hesitation when accelerating, had power if pushed. Also around the same time the cruise control would kick off either with acceleration up a hill or if you hit a bump in the road. Recently had the check engine light on, came up with a P0300 code. Changed the spark plugs, wires and fuel filter. Almost feels worse now, is happening more often. In the past there was a very loud sound from under the engine like something fell out. Someone mentioned it may be the blow out plug for the catalytic converter. Other than that the gas mileage sucks, tried a gas additive, car has a slight leak in the gas line, but I doubt this is the issues (just water in the line.) Yesterday it clunked real hard twice like a transmission issue, not sure if that is the misfire or related to that over the engine. Help!