So this decouple the physics from the render rate doesn't work so good. You get a really annoying jitter.
So I give up. I'm just going to lock the whole game loop to 60 fps. Runs a lot smoother, although I've only got a 60hz monitor to test it on. Hopefully it won't look too bad on an 80hz.
And if your computer can't manage 60fps, it is below the minimum spec for the game.
Don't know what else to do really.