Engine misses at idle and throughout driving range, OBD code P0300

Van drove fine yesterday morning anf then in the afternoon it ran really rough, and the CEL light came on. I read the codes P0325, and P0306. I check the fuel injector in the 6th cylinder and it was not functioning properly replaced it, and now I ge the same rough idle and missing through driving range, but the code is now P0300. What could it be and where do I start. I have recently replaced the cap, rotor, spark plugs, wires, and distributor.