2D Sprite shadow artifact when using Spotlight

Hey guys,

I am doing a experiment where my sprites are layout in 3D space and I am applying a shader to force them to cast and receive shadows. This works great when I am only using a Directional Light, but as soon as I have a Spot Light, all the sprites get weird artifacts, black lines (shadows are still casted normally). Attached is a side by side comparison between Directional and Spot Light (Closed-up look below).

Does anyone experience this before, or know why this is happening? I know that if I use a Quad instead of the Sprite everything works fine, but I really wanted to keep using Sprites since I can use my spritesheet animations (not quite sure if I can do that in a regular MeshRenderer).

This is the shader I am currently using in my Sprites (provided by @anlev):

Code (CSharp):

1. Shader "Sprite/SpriteShadow" {
2.     Properties {
3.         _Color ("Color", Color) = (1,1,1,1)
4.         [PerRendererData]_MainTex ("Sprite Texture", 2D) = "white" {}
5.         _Cutoff("Shadow alpha cutoff", Range(0,1)) = 0.5
6.     }
7.     SubShader {
8.         Tags
9.         {
10.             "Queue"="Geometry"
11.             "RenderType"="TransparentCutout"
12.         }
13.         LOD 200
14.  
15.         Cull Off
16.  
17.         CGPROGRAM
18.         // Lambert lighting model, and enable shadows on all light types
19.         #pragma surface surf Lambert addshadow fullforwardshadows
20.  
21.         // Use shader model 3.0 target, to get nicer looking lighting
22.         #pragma target 3.0
23.  
24.         sampler2D _MainTex;
25.         fixed4 _Color;
26.         fixed _Cutoff;
27.  
28.         struct Input
29.         {
30.             float2 uv_MainTex;
31.         };
32.  
33.         void surf (Input IN, inout SurfaceOutput o) {
34.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
35.             o.Albedo = c.rgb;
36.             o.Alpha = c.a;
37.             clip(o.Alpha - _Cutoff);
38.         }
39.         ENDCG
40.     }
41.     FallBack "Diffuse"
42. }