My 2005 Ranger with a 3.0 has a misfire, P0303. When the engine is cold it starts up and runs great, after I drive it a while and get into stop and go traffic it has a miss and eventually the check engine light comes on. So far I have replaced the following items: Fuel pump and filter Coil pack ( twice to make sure ) Plugs and wires # 3 injector Crankshaft position sensor Throttle body gasket Plenum gaskets PCV valve Cleaned MAF sensor Spark tested # 3 Checked injector wiring harness with a noid light Compression test and # 3 is very close to the same as other cylinders. Cleared the engine control computer. The PCV valve was hung open and completely worn out, I smoke tested it and I had a bad vacuum leak at the throttle body gasket. After replacing the PCV and throttle body gaskets it runs better but after driving 50 miles it starts idling rough and eventually I get the P0303 engine code telling me that # 3 cylinder is misfiring. Could it be weak valve springs on # 3?