Thank you for your replies. I can give a short update to my problem.
After I wrote my last post, I sliced and saved my model again, cleaned the extruder wheel, the build plate and did every possible maintenance. I assembled the extruder stepper again and tried to print my model. To my surprise it seemed to work at this point.
After approx. 6 hours of printing my problem was back. While stuttering I checked that the roll is spinning easy, I twisted the cable again, everything without effect. I even additionally pushed the filament by hand, but the extruder did not stop stuttering at some points. The only thing I recognized was, that the extruder was quite hot.
Is it possible that there occur problems with single phases of the stepper due to increasing resistance because of the heat?