I agree with the low miles part. 40k is a very low mileage to start having those issues (first for everything though). I have had cars come in that had bad gas due to excess water getting into the tanks and reported the same conditions you did. Your mechanic probably did the fuel system cleaner to make sure while also putting in fresh gas. Most gas doesn't come from each individual company, it is all in a pipeline that gets pulled out at the stations, mixed with each stores additive pack, and then branded as their own. So your costco gas is essentially the same as the Chevron, with a different additives added in. I would try switching gas stations for a while and see if that helps. Also may be worth letting the manager at Costco know, dont expect compensation, just a friendly email/letter explaining your situation. It may be a issue that they don't know about.