Wireframe material overlapping with mesh

I'm trying to get a mesh to display a wireframe outline in the game view just like how it works when you select an object in the scene view. The problem is that when I use GL.LINES to draw the wireframe in the game view, the wireframe material overlaps with the mesh's materials. Here's a picture to illustrate the issue (open in new tab for higher res):

As you can see, the lines sorta cut off in certain areas. They also flicker when I move my camera around in game (z-fighting?). I tried adding/messing with an offset value in the shader but it doesn't seem to help (or I was just using it wrong). This is the shader I'm currently using for the GL.Lines material:

 Shader "Wireframe"
 {
     Properties
     {
         _Color ("Color", Color) = (1,1,1)
     }
     
     SubShader
     {
         Color [_Color]
         Pass {}
     }
 }

Here's the code that creates the wireframe on a mesh when you click on it: http://pastebin.com/n1WQedpA Add the script to the main camera if you want to test it out.