I have a CEL from a bad gas cap also (no hiss when you open it) and my idle has been fine even tho this is my 7th CEL in the last five months (it goes on for anywhere from a few hours to a few days and then goes out for a couple of weeks).
The fuel injector cleaner is a good idea as long as you don't go overboard with it. If you notice any improvement you're on the right track. If not, I'd go back and look at the electrical (bad battery connections or possibly even an dying battery could be the cause.)
