Well, your mpg is certainly good, so it sounds like nothing is "way" off. I've seen reports that those plugs need to be changed more often than some others, but still should get much more than 5K. How about the battery - still the original, or has it been replaced? Not very likely to be that, but week batteries are known to cause all sorts of weird electrical things. One other possibility that comes to mind is the IAC. These things can be a PITA to figure out, especially with no codes.