Ho risolto in questo modo:

Ho inserito una variabile intera e un Timer impostando l'intervallo a 100 :
codice:
Timer1.Interval=100
Dim tempo as Integer=0
poi ho aggiunto questi eventi:
codice:
Private Sub Timer1_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles Timer1.Tick

        If tempo = 1 Then Me.Refresh()
        tempo += 1

    End Sub

    Private Sub RichTextBox1_MouseWheel(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles RichTextBox1.MouseWheel

        tempo = 0

    End Sub

    Private Sub RichTextBox1_VScroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles RichTextBox1.VScroll

        tempo = 0

    End Sub