How to use an external file that lists URLs - Singleton

Hi,
In my quest to make my ANTS load testing a bit better, I'm trying to use a list of URLs from a currently-live webserver. Here's what I've done:

-Looked at our clients web stats and found a busy day
-parsed the resultant web logs from IIS and extracted all resources, one line each
-put this list of resources into an external file of >1,000,000 lines
-tried to implement a Singleton class to read the file and sequentially feed the resource URLs one-by-one to ANTS virtual clients. The singleton is supposed to be instantiated ONCE and reused by each of the threads - so that I only have to read the big URL file once, and then it's in memory for subsequent reads.

Here's the problem:
I can't get the Singleton to work as expected. I'm not very experienced with VB (but I'm a whiz with C#), but in looking at tutorials and examples, I'm pretty sure I'm *close* to doing what I want. The problem is that I seem to be getting thousands of Instances of the Singleton created, instead of just one. Because I am getting multiple instances, I am having the URL source file read thousands of times (concurrently!), and am getting the following error messages:

Exception message: The process cannot access the file "C:\tmp\test.txt" because it is being used by another process. - 3782 times
Exception message: Object reference not set to an instance of an object. - 182 times

(and these messages are only from a 15 second run!)

So, does anyone have any idea where I can go from here?


Code for my quasi-singleton:
===================

Imports System
Imports System.Collections
Imports System.IO
Imports System.Runtime.CompilerServices
Imports System.Threading

Public Class FileReader
Dim logfile As String = "C:\tmp\antslog.txt"
Dim sourceFile As String = "C:\tmp\test.txt"
Private _alURLs As ArrayList ''holds a list of URLs
Private _URL_counter As Int32 = 0 ''counter to increment through the array
Private Shared ReadOnly padlock As Object = New Object

Private Shared FInstance As FileReader = Nothing

Private Sub New()

End Sub

Public Shared ReadOnly Property Instance() As FileReader
Get
SyncLock padlock
If (FInstance Is Nothing) Then
FInstance = New FileReader
End If
FInstance.WriteLog("getting Instance")
Return FInstance
End SyncLock
End Get
End Property

Private Sub ReadList()
WriteLog("ReadList")
''first test to make sure that the URL list has info in it
If (_alURLs Is Nothing) Then
''populate the array
Dim din As StreamReader = New StreamReader(File.Open(sourceFile, FileMode.Open))
Dim str As String

' loop through all the rows, stopping when we reach the end of file
Do
str = din.ReadLine()
If str <> Nothing Then
_alURLs.Add(str)
End If
Loop Until str = Nothing
End If
End Sub

Public Function GetURL() As String
WriteLog("Getting URL")
ReadList()
''return the next URL in the list, but if we are at
''the end of the array, move to the beginning
If (_URL_counter = _alURLs.Count) Then
_URL_counter = 0
End If

Return _alURLs(_URL_counter).ToString()
End Function

Public Sub WriteLog(ByVal text As String)
SyncLock padlock
Dim objReader As StreamWriter
objReader = New StreamWriter(logfile, True)
objReader.Write(DateTime.Now & " - " & text & System.Environment.NewLine)
objReader.Close()
End SyncLock
End Sub
End Class