i'd try a different station then, rather than brand, and see if it makes a difference....
reason-being, my uncle used to own a gas station. and, from what he told me, gas stations normally buy gas from their suppliers. but, if they run low and the delivery supply schedule cannot be met, they'll buy from anywhere in distribution to keep the pumps rolling. so, in theory, if u roll into a Unocal station, you may or may not be getting Unocal gas, and the same with all other stations. it just depends on how efficiently -that- particular station manages its gas inventory.
i used to use Unocal 91 from the same gas station in alhambra... my bimmer would get check engine lights every now & then. so, then i switched to a Chevron 91 at a different station, and i've never had that issue anymore. i fill up both my cars and the 6r with Chevron 91 at that station consistently nowadays... no more issues.
it might be shady, corner-cutting gas station, cuz i can't imagine all chevron gas being bad. that said, stay away from arco. i swear they mix a ton of alcohol in their gas, cuz during college i used to use arco and for the same amount, it burns up almost 1.5xs as fast.