If a $9 bottle of fuel additive solves the problem, then I guess you've got the solution. Fuel doesn't go stale in two weeks though. Maybe check your fuel cap to make sure it's sealing properly? I don't even think that would affect the fuel that way but, just trying to think of how fuel would be affect that way so quickly???
From the original post describing the issue, it sounded more like a jetting issue.
Would the fuel additive mask the issue of a jetting problem?