Till now I have given 2 times ielts and have scored 7 bands in each. I have been stammering since childhood. According to my experience, you should always communicate with the examiner about your stammering. You must tell him/her that he/she should not judge the speed and articulation of your speech. This actually makes it easier for the examiner to judge on other aspects and don’t deduct marks due to low confidence. Also the examiner may get little lenient in giving marks because for a stammerer it’s very brave decision to give a speaking exam. Along with this you should focus on your calmness and should have practice in making answers lengthier at the spot. Do not ever stop your answer until the examiner asks to stop and to proceed to the next question. Have a smile on your face and be gentle. Rest leave everything to god or nature and eventually you will clear it.