Did you do anything to sort the rpm fluctuation?
Not really, well unless you include swapping my fan to channel 4 from channel 3. It initially varied for a minute or so after boot but has now settled down and is more or less running at a constant speed. The speed still varies sometimes between 800, 900 and 1000 RPM but doesn't wind up and down, which was frankly quite annoying.
Just waiting on Phoebetria being developed to adjust fan curves so I can reduce the idle speed of the rear fan to my liking. I'd prefer it to be at a designated speed at a designated temperature rather than this random speed I seem to be getting.