Interface And Graphics :: Cutting Text From RichTextBox?

I want my Richtext box to be populated with a text string that contains HTML-style tags to set colour, underline etc.When I try to remove the tags using .cut(), the first cut always works, the second cut only sometimes.Here's a sample to test it:Create a new project and add a button and a richtext box.Add the following code:

Code:
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim testString As String = "ABC<R>DEF</R>GHIJKL"

[code]....

This code works fine.The text between the tags is turned red, then the tags are removed.Now rem out the msgbox line and run again.Sometimes it works, sometimes the second cut fails with no message or warning.I am using Visual Basic 2010

Visual Studio 2010 v 10.0.30319.1
.NET framework v 4.0.30319