Code P0300 random misfire

My car is the X model. It idles very rough at all times and when you put it in gear it stalls the engine light flashes on and off then stays on, I changed the spark plugs and I tested all the coils, I even tried new cam shaft sensor still the same thing. P0300 is a random misfire and I took it to a shop after I gave up and they cannot even figure it out!