I have one that goes from 4k to 6k every few minutes - or at least it did when it was SLIGHTLY warmer than it is now. (I mean by 2-4 degrees) I upgraded to "File System Version Fri Nov 17 17:57:49 CST 2017" in hopes of it correcting, but it didn't.
The highest chip temp is currently 76 and it no longer ramps up and down - however, I have no doubt that when the temps get back up to 78 it will resume the up and down.
What is your highest chip temp on your S9 that is spinning at 6k?
What does it matter that the fan in ramping up and down. I have one that does that as well and have seen multiple posts be individuals attempting to fix it. I have never had an issue with the machine and have never bothered with it. Is it because the sound? Worried about extra running cost with electricity?
The only problem I have with the issue is the fan may die while I sleep and I end up burning my boards, I have seen multiple posts, but not a sure fix. It will add extra cost, but mainly just it not running 100% as it should, I'm already ocd enough and different board temps are enough, hahaha. As long as the boards stay cool, my worry is just the wear and tear on the fan. Still learning