Gasoline in the US is fungible - everyone fills the pipeline and takes whatever comes out at the other end. The difference is in the additive package, which is only injected at the rack; in other words, as the tank truck is being filled. It would be an expensive logistical problem to segregate gasoline made at a certain refinery and distribute it to only one brand of gas station and keep everyone else from getting it also.
Having said that, sometimes it is still done. I believe Amoco premium (advertised as "clear") and perhaps Sunoco 94 and higher octane fuel would have segregated supply routes.
What does change about 15 times a year is the volatility - how easily the gasoline vaporizes. The volatility is seasonally adjusted, by state law, to accomodate the region's typical ambient temperature. Therefore, while every state will adjust volatility levels, the southern states will all have less volatile fuel than the northern states.
It is possible that driveability problems occur when the gasoline requirement changed recently and you have a sudden and unusual temperature swing. The small amount of additive (typically 0.005% or less) won't have such an immediate effect. The detergents would take a tank or two or more to make a difference.
