Misfires must be diagnosed. As you can see, randomly changing parts in an attempt to diagnose the problem can be vert COSTLY. Besides all the parts that you have allready replaced, there could be a problem with the fuel injectors, there could be cylinder compression problems caused by a number of possible causes including bent/burnt valves, broken valve springs, or a head gasket blown between cylinders. There could be timing problems due to erratic knock sensor input. Fuel shutoff due to throttle position sensor "dropout". "Random" misfires (which is what a code P0300 code is) can even be caused by something as crazy as a vacuum brake booster leaking and creating a large vacuum leak....the list goes on and on....

It is easiest and most cost effective way to diagnose this type of problem is by using a scan tool and looking at all of the engine data and/or taking a "snap-shot" of the data when the problem is occuring. This way, you can tell which parts of the engine control system are being affected so those areas can be inspected for possible failures. The first step is to isolate the causing SYSTEM. So far, you have replaced parts for the fuel system, the ignition system and the EGR system....