Created new world in 13w02a bouncing up and down like there is no tomorrow

So i created a new creative world in the latest snapshot, upon joining, i'm right next to a tree bouncing up and down like there's no tomorrow.

My Y coord keeps changing from 64.874 to 64.954 and it hasn't stopped since i created the world about 5 or so mins ago.

I can't move away from that spot, and when i disconnect and reconnect it just goes back to doing the same thing.

If i try to use the mouse to move, it is all jittery too but it does let me look around 360.

My in-game FPS is about 300.
My chunks number on F3 switches so fast between 100 and 150 it just isn't funny too.