Ford Fiesta Engine Problems

As you state Ford recommends 91-octane fuel for your car, so you would expect it to run on regular unleaded without a problem. I assume you have tried E10 unleaded, which generally has an octane rating of at least 94. If not it might be worth trying that, but   other than that go back to your dealer and point out to him that   you're not happy with your car and that you want it fixed.