My 06 Roadster had a rough idle and it took the dealer a while to determine the location. It was a vacuum line and they told me it was under the engine cover. I have never had an idle issue since the fix and I will tell you the difference is night and day! So; if you think you have a rough idle problem; you have a rough idle problem. I have not noticed a difference in the idle due to gasoline manufacturers. I'm using premium grade primarily from Marathon, Sunoco and BP.
I hope this helps!