I just did a search on here with the phrase "rough idle" and found quite a few. Look under
check out the "cold starting" thread.
With 15k miles and if it worked fine before and now it doesn't, process of elimination says something happened. I know **************** happens sometimes but probability says it was them and not chance. I doubt you all of a sudden got carbon build up. It sounds like something computer or system related. Possible o2 sensor. I changed my plugs(probably first time) at 60k miles and that fixed my slight idling problem.
Good luck and don't take no for an answer