If you're getting stumbling with either 87 or 91 I'd say there's something wrong worth checking. I'm a new SE owner so don't know all the details of the engine operation of our vehicles.
In most new cars if the crank senses a difference in the amount of power from cylinder to cylinder the ECU changes operation. A common source of this problem is misfire. Plugs are a suspect and they are the cheapest to check and fix. A typical sign of plugs misfiring is stumbling at low RPM which smooths out at higher RPM.
Have you changed the plugs or wires recently? If so, did you switch from a colder to hotter or hotter to colder plug? Have you pulled the plugs to give them an eye?
Again, I'm a new owner of this vehicle/engine so I'm not up on its actual operation but this would be my first suspect.