My 2008 Chevy Cobalt has a CEL - code P0303. I've changed spark plugs and coil packs, but that hasn't fixed it. The car runs okay for the most part unless I am sitting at a stop light or idling somewhere, then it idles roughly and the CEL light starts flashing. I'm not a mechanic, but am trying to see if myself, since I can't afford to take it to the shop. Anyone know what could be wrong, or what I can check?