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Then something is wrong with some of it. If those are the only codes, then it really has to be a secondary ignition problem. It could be a bad coil or something wrong with the parts already changed. Have someone inspect the inside of the distributor cap and test the coil with an ohmeter. the codes mean
PO300 - Random misfire
PO302 - Misfire on cyl #2
PO306 - Misfire on cyl #6