That probably just means that you have your A/C system set in "AUTO" mode. If you want the fan to blow at a steady speed, just set the fan speed via the "manual fan speed control" button in the middle console, and the speed will not vary - until you push the auto button again.

Also, don't forget to adjust the "dial" up near your center vent duct from hot to cold depending on what you want. It seems to impact the temperature MUCH more than the actual digital temperature selector does! I still haven't figured out why BMW keeps these 2 separate control functions because it only causes confusion!