Intermittent "Load Report Failed" error on IIS6 ASP 4.0

Question

I can run reports designed in Crystal 2011 and embedded in VS2010 designed web pages on a Windows 2003 SP2 server every day.  However, once every 2 - 3 days I get a "Load Report Failed - A document processed by the JRC engine cannot be opened in the C++ stack".  The first instance of this error causes all subsequent user access to Crystal Reports to fail with the same error.  When the ASP 4.0 application pool is recycled the error goes away.
I was getting the "Load Report Failed" error due to "out of memory" I until made sure that the crystalreportviewer.dispose was run on exiting each report page. 
Here is  how I run Crystal Reports:
   Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init
        If Not IsPostBack Then
            Dim myAL As New Hashtable
            myAL.Add("MO1", Request.QueryString("MONum1"))
            myAL.Add("MO2", Request.QueryString("MONum1"))
            Dim filename As String = ""
            filename = Server.MapPath("~/reports") + "\MfgPicklistByMO.rpt"
            Dim u As New CrystalUtilities
            Session("crViewer") = u.SetupReport(filename, myAL)
        End If
        CrystalReportViewer1.ReportSource = Session("crViewer")
    End Sub
   Public Function SetupReport(ByVal fileName As String, _
                 ByVal Params As Hashtable, Optional ByVal dbParam As Boolean = True) As   CrystalDecisions.CrystalReports.Engine.ReportDocument
        Dim objCrystalReportDocument As New CrystalDecisions.CrystalReports.Engine.ReportDocument
        objCrystalReportDocument.Load(fileName)
.... logs onto a selected database, and loads paramters

Answer 1

I have installed CrystalReportViewer1.Dispose() statements on all of the link events to navigate out of the pages with the CrystalViewer.  However, it turns out that several users are using the back arrow to navigate out of the page instead of the navigation links I have put on the page.
I can't seem to find any event in ASPNET 4 that fires when the back arrow is pushed.  If I could find such an event I could put a CrystalReportViewer1.dispose statement there assuming CrystalReportViewer1 still has scope.    The Page_Disposed event does not fire when the back arrow is pushed.
Environment:  VS2010, Crystall 2011, ASPNET 4.0