Flickering mesh

Shader noob here.

I wrote myself a billboard shader:

Code (CSharp):

1. float4 vertex = i.vertex;
2. float2 camera = mul(unity_WorldToObject, float4(_WorldSpaceCameraPos, 1)).xz;
3.  
4. float cosa = dot(float2(0, -1), camera) / length(camera);
5.  
6. float direction = dot(float2(-1, 0), camera);
7. float angle = sign(direction) * acos(cosa);
8.  
9. vertex = rotate(vertex, angle);
10.  
11. o.vertex = mul(UNITY_MATRIX_MVP, vertex);

This works very good except for the flickering.


I also applied a script to the meshobject which uses

Code (CSharp):

1. transform.LookAt(camera)

so i can reliably reproduce the error i saw occasionally which appeared when the objects normal was directly pointing at the camera. That's when the mesh starts flickering. It's probably because of how i calculate the delta angle but I'm kind of lost.

The error:
https://giphy.com/gifs/l3vRmutGsDjSFqeLC

What bugs me is that the flickering also appears when just rotating the camera, although i never use the camera rotation in my calculations.