Vertex displacement shader troubles

Hi there!

I want to write a surface shader that does vertex displacement and unculled cutout shading.

The code is very simple, as you can see below. There's a vertex function for the displacement and a surface function for the shading.

Unfortunately the shader behaves wrong when it's asked to cast shadows. It seems it takes the solid, un-deformed geometry aswell as the deformed one for shadow calculations. The two sometimes even cast shadows on each other. (see the screenshots).

How can I get only the deformed cutout part to cast shadows?

Thanks!

 Shader "Transparent/Cutout/Diffuse Wiggle" {
 Properties {
     _Color ("_Color", Color) = (1,1,1,1)
     _MainTex ("_MainTex", 2D) = "white" {}
     _Amount ("_Amount", Range(0,.2)) = 0.1
     _Direction ("_Direction", Vector) = (0,0,0,0)
     _Scale ("_Scale", Range(0.1,10)) = 1.0
     _TimeScale ("_TimeScale", Range(0,10)) = 1.0
     _Cutoff ("_Cutoff", Range (0,1)) = 0.5
 }

 SubShader {
     Tags { "RenderType" = "Opaque" "Queue" = "Transparent" }
     Cull Off

     CGPROGRAM
     #pragma surface surf Lambert vertex:vert addshadow alphatest:_Cutoff 

     struct Input {
         float2 uv_MainTex;
     };

     half4 _Direction;
     float _Amount;
     float _TimeScale;
     float _Scale;
     sampler2D _MainTex;
     half4 _Color;

     void vert(inout appdata_full v)    {
         // calculate offset amount
         float4 offs = float4 (normalize(v.normal), 0) * sin (_Time.y * _TimeScale + (v.vertex.y * _Scale)) * _Amount;

         // offset vertex
         v.vertex += offs * _Direction;
     }

     void surf (Input IN, inout SurfaceOutput o) {
         half4 c = tex2D (_MainTex, IN.uv_MainTex);
         o.Albedo = c.rgb * _Color.rgb;
         o.Alpha = c.a * _Color.a;
     }
     ENDCG
 }
 Fallback "Transparent/Cutout/Diffuse"

}