Spirometery General Question

When doing spirometery I find that I can do the breathing normal for a couple of cycles, then the fast breath out alright but then when I get to the end of the breath out I find that I cough like mad and have to stop. This affects the later part of the readings. I am fairly consistent with my readings over each test both before and after ventolin but wondering if anyone else has had this issue and was there a way of overcomming it....