Getting inconsistent Error 91-Object variable or With block variable not set

[mfq]

I'm getting Error 91 inconsistently when I run my macro. Macro stops
when it reaches the "USERNAME" line. I've tried setting the object to
nothing at the end and also using a fully qualified name. None works.
Anybody have any ideas? Does this have something to do with the browser
object? (I'm using Excel 2000)

Here's my code:
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Public Sub FHLBSF()
Dim dFRC(1 To 13, 1 To 3) As Double
Dim ie As Object, ipf As Object, tmpWksht As Object
Dim dDuration As Double, PctDone As Double

' Prepare to open the web page
Set ie = CreateObject("InternetExplorer.Application")
ie.Visible = False

With ie
.navigate "https://member.fhlbsf.com/member/admin/login.asp?"
Do Until Not .Busy
DoEvents
Loop
End With

' INCONSISTENT ERROR 91 HERE
ie.document.all.Item("UserName").Value = sUName
' ERROR
ie.document.all.Item("Password").Value = sPW
ie.document.all.Item("submitButton").Click

ie.navigate sStandardVRCURL
Do Until Not ie.Busy
DoEvents
Loop

ie.ExecWB 17, 2
ie.ExecWB 12, 0

'Logout
With ie
Set ipf = ie.document.all.Item("menu5")
ipf.Click
End With

ie.Quit
Set ipf = Nothing
Set ie = Nothing
End Sub
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