Subscript out of range when trying to set worksheet variable

i have the below which sometimes work and sometimes does not.  It stops at the Set wks line.  This module is in an ACCESS 2003 database opening the Excel workbook.

Sandra

Option Explicit
'Daily upload of debits and credits from accounting
Const strPath = "\\Sf1\User1\shared\EFT\ATM\Private\Backend\ATMUpload.xls" 'Path to directory
Const strTab = "Cumulative Journal Transaction " 'Name of data source tab
Const strWKBName = "ATMUpload.xls"
Public Sub DefineDataRange()
On Error GoTo ErrorHandler
'Late Binding (Needs no reference set)
    Dim objExcel As Object
    Dim wks As Worksheet
    Dim wkb As Workbook
    Dim intRow As Integer
    Dim rngDataSource As Range
   
     '   Create a new Excel instance
    Set objExcel = CreateObject("Excel.Application")
     
     '   Open it
    With objExcel
        .Visible = True
        .Workbooks.Open (strPath)
        .Workbooks(strWKBName).Activate
        .Workbooks(strWKBName).Worksheets(strTab).Select
       
 'SOMETIMES RUNS SOMETIMES DOESN'T-Subscript out of range error
Set wks = Workbooks(strWKBName).Worksheets(strTab)
    End With
   
    With wks
        .Range("A5").Activate
        intRow = .Range("A200").End(xlUp).Row
    Set rngDataSource = Range("A5", "G" & intRow)
        rngDataSource.Name = "DataSource"
    End With
   
    Workbooks(strWKBName).Close SaveChanges:=True
     
Exit_ErrorHandler:
    Set objExcel = Nothing
    Exit Sub
     
ErrorHandler:
    objExcel.Visible = False
    MsgBox "Error Number: " & Err.Number & vbCrLf & _
           "Description: " & Err.Description
    Resume Exit_ErrorHandler
End Sub