C# - Random Number Generator Returning Zeros?

I have an ASP.NET application that relies on the Random class to generate a pseudo-random string. It uses the following code (this is part of a larger piece of sample code provided by Google for apps SSO):

public static class SamlUtility
{
private static Random random = new Random();
private static char[] charMapping = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p' };
public static string CreateId()
{
byte[] bytes = new byte[20]; // 160 bits

[Code]...

This normally works very well, but from time to time it starts generating a string of 'a'. From what I can tell from debugging, Random simply stops returning random numbers and instead fills bytes with the same value over and over. I've patched this by using a GUID instead, but I'm curious what happened in the original code. I'm assuming some form of entropy exhaustion, but I can't find any reference in the docs. Also, each time this has happened, performing iisreset restored the correct behavior.