The mouse Y-axis, meaning the up-down movement of the mouse changes constantly depending on how high or low your frame per seconds is. If it's very high ( Usually in caves, dungeons and instanced areas), the up down movement will skyrocket. If the FPS is low ( Mostly in towns with alot of NPCS, atleast for me) , the up down movement will go down dramatically. Out in the world of Skyrim this changes all the time depending on your FPS and It's fairly noticable.

I'd say this comes pretty close to a game breaking bug. It's playable, but it can annoy you pretty quickly and can make things alot harder for you, especially if you're an archer like me.
Why on earth does it do this? Is there a permanent fix that solves all? ( I've tried a good few, including the frame rate limiter, but it doesn't solve it, It just reduces it a little)

When can we expect a patch for this, if even at all?

Yes, exactly it depends from fps and mouse acceleration =0 do not work whaever players said. If someone know over solution, post please