idling problem

What is your question?

I have a 2003 4 cylinder, manual trans, 116k miles. for the most part, been a good vehicle. as of late it idles very roughly. it has had a tune up and a fuel filter replacement, still not getting any better. the SES light will come on and then go off again at times. i was told it is a build up of carbon from poor quality gasoline, so i should switch to a higher quality gasoline and it will correct itself in time... it has been several months with the higher quality gas, and it is not corrected. when the car is idling, the RPM's are less than 1 and it shakes and sputters, but as of yet, hasn't stalled.