Two options.
1. Go into BIOS Setup where your CPU_FAN header is configured. Choose to have it run under Manual, not Standard. Then set the specific speed you want. Make sure it is so high that it will never allow your CPU to overheat. When finished, SAVE and EXUT.
2. Instead, at that same place, create a custom fan response curve. What I suspect is happening is that your system often is running just on the border temperature between two fan speeds. So you could change the fan response curve to get it to run a bit faster at slower speeds, and maybe get it set so it does not keep changing so often.