I get my gas from the cardlock in town cuz its cheap but I get symptoms like yours if I use their premium gas. Evidently no one else does and it sits and gets water in it.
Edy from "Behind the Redwood Curtain"

85 4Runner 22re, mostly stock. Over 340K miles(SOLD for $4000 7/06)

98 2.7 4cyl 4Runner Bone Stock perfect 200,000 miles (alright, it has a HG and clutch in it's past)