Basic replacement shader causes objects to flicker

Hi,

I'm in the process of learning all I can about Unity's shader support. I found a simple shader example here (http://docs.unity3d.com/Documentation/Manual/ShaderTut2.html), but when I replace my main camera's renderer with it, I get strange flickering with the roads in my scene.

This first image seems ok...

However, if I move the camera just slightly, the roads flicker in/out. The image below shows that the left half of the scene's roads are gone, but it's more random than that.

Any thoughts as to what might be going on? If I comment out the call to SetReplacementShader, everything renders properly.

Here is the C# code that replaces the shader.

 using UnityEngine;
 using System.Collections;
 
 public class ShaderReplacer : MonoBehaviour 
 {
 
     // Use this for initialization
     void Start () 
     {
         GameObject go = GameObject.Find("Main Camera");
         Camera c = go.GetComponent<Camera>();
 
         Shader s = Shader.Find("Custom/MyFirstShader");
 
         Debug.Log("Camera name:" + c.name);
         
         if (s == null)
         {
             Debug.Log("Shader could not be found");
         }
         else
         {
             c.SetReplacementShader(s, "");
             Debug.Log("Shader name:" + s.name);
         }
     }
     
     // Update is called once per frame
     void Update ()
     {
     
     }
 }
 

... and the shader code itself

 Shader "Custom/MyFirstShader" 
 {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,0.5)
     _MainTex ("Texture", 2D) = "white" { }
 }
 SubShader {
     Pass {
 
 CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 
 #include "UnityCG.cginc"
 
 float4 _Color;
 sampler2D _MainTex;
 
 struct v2f {
     float4  pos : SV_POSITION;
     float2  uv : TEXCOORD0;
 };
 
 float4 _MainTex_ST;
 
 v2f vert (appdata_base v)
 {
     v2f o;
     o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
     o.uv = TRANSFORM_TEX (v.texcoord, _MainTex);
     return o;
 }
 
 float4 frag (v2f i) : COLOR
 {
     float4 texcol;
     texcol.rgb = tex2D (_MainTex, i.uv).r;
     texcol.a = 1;
     return texcol * _Color;
 }
 ENDCG
 
     }
 }
 Fallback "VertexLit"
 }