To answer the questions above about the Impala, I don't know how it was previously driven because it was owned by the US Government. Usually their GSA auction cars are super high mileage (usually law enforcement) or they are gently driven by bureaucrats and have few miles. The information about average MPH etc could be extracted from the computer if it wasn't reset, but I forgot to do that when I got it so any data on it would be after I drove it for 9 months and added 50% more to the mileage.
It's a 6 cylinder vehicle but switches to 3 cylinders when the computer says it gets better gas mileage that way. The problem happens when its running on 6 cylinders, I haven't tested to see if it also happens at 3 (basically highway speeds). It's also an E85 so engineered to take a very wide range of octanes and fuel mixtures. I'm running it on regular unleaded (E10). I did try adding some PRI-G to the fuel, because the problem started after I fueled at Safeway for the first time in many years (and first time ever with this vehicle) and the problems were reminiscent of when I was driving through the Yukon and our Ford Explorer started stuttering similarly, and it was fixed by adding a can of STP to the tank. Every time I refueled it did the same thing, so I was scrambling to find STP in the middle of nowhere and all the fuel additive shelves were bare. The problem disappeared when I got to Alaska. They just had really bad gas in the Yukon Territory back then. But I also realize that fueling at Safeway was probably just a coincidence and the PRI-G didn't help. I've had good luck treating old/bad gas with small engines with PRI-G.
It's a 6 cylinder vehicle but switches to 3 cylinders when the computer says it gets better gas mileage that way. The problem happens when its running on 6 cylinders, I haven't tested to see if it also happens at 3 (basically highway speeds). It's also an E85 so engineered to take a very wide range of octanes and fuel mixtures. I'm running it on regular unleaded (E10). I did try adding some PRI-G to the fuel, because the problem started after I fueled at Safeway for the first time in many years (and first time ever with this vehicle) and the problems were reminiscent of when I was driving through the Yukon and our Ford Explorer started stuttering similarly, and it was fixed by adding a can of STP to the tank. Every time I refueled it did the same thing, so I was scrambling to find STP in the middle of nowhere and all the fuel additive shelves were bare. The problem disappeared when I got to Alaska. They just had really bad gas in the Yukon Territory back then. But I also realize that fueling at Safeway was probably just a coincidence and the PRI-G didn't help. I've had good luck treating old/bad gas with small engines with PRI-G.