I can't picture the fuel filter, which is part of the fuel tank module inside the gas tank, causing a drivability issue with 87 octane and not with 89 or higher octane. As far as top tier gasoline use, I don't know what she uses. However, if valve deposits were the issue, I would expect the issue to not be quickly rectified by running a tank of 89 or higher octane fuel through it. It takes time to build up valve deposits and they don't get removed quickly from the valves unless they serviced by some kind of cleaning method, be it chemical or by walnut shell blasting. Thanks for the suggestions, though!