Stuttering during rapid acceleration
 

To keep it short, I have a US spec Mini Cooper and have had it in Europe for almost two years.

About eight months ago, I noticed the car would stutter a little, almost like running over continuous small pot holes in the road. So I started adding octane booster thinking the gas on base was a little low on the octane side. However, for the past few months, whenever I go to rapidly accelerate to pass a car, it will suddenly stutter several times, sort of like you just tapping the break several times.

I took it to the BMW dealer I have nearby and a mechanic took it for a little spin and the car acted up with him behind the wheel. So the dealership kept the car for the day to run tests and came up with practically nothing. They told us we were using the wrong gas in it. I'm using 91 octane which is available on base with octane booster each time I fill up.

Unfortunately, I'm not available to go back to the dealership I bought the car from for a bit over a year. So I'm looking to see if anyone else has had a similar problem or other advice. I do have a secondary dealership I can drive too that is a bit further and I've heard they are better with cars than the closer one to me; but I don't know. I just feel like there's something wrong.