SRT 6 Roasdster, I had the same codes back about six months ago. I was told PO 300 was multiple misfires and 301 thru 306 indicated individual cyclinders. My coupe would run rough under accelleration around 90 mph. I checked with Rob at Needswings. First check your plug wires to see if you have any loose connections. Then check your plugs for fouling. I ended up getting new plugs and magnacor wires and it solved the problem.