Right Fax Error "Problem Converting Fax Body or Cover Sheet"

Below is the code I use to send the fax.  It works 60% of the time.  When it doesn't work, I get an error from Right Fax that says "Problem Converting Fax Body or Cover Sheet" with the data "Error 2 occurred while trying to load the images for the fax.  The system cannot find the file specified."  The files I attach are left on the drive for an hour after faxing.  They all open properly.  All of the faxes include one text file and one pdf.  All of them are identical in my testing, but only 60% are sent and 40% return this error...

I have tried adding thread.sleep calls in various places, but even with sleep times of 10 seconds after attaching and after sending, the error occurs.

Here is my code...

                RFXServ.OpenServer()
                RFXDoc = RFXServ.CreateObject(RFCOMAPILib.CreateObjectType.coFax)
                With RFXDoc
                    'Populate the basic data... we already know that we have a non-null fax number,
                    'so no need to check again...
                    .ToFaxNumber = dr.FaxNumber
                    With RFXServ.AuthorizationUser
                        'Effectively, we do not want any feedback coming back from the fax server telling
                        'us anything about the sending status...
                        .SendNotifyOnIncompleteFirstTime = False
                        .SendNotifyOnIncompletePeriodically = False
                        .SendNotifyOnNoHoldForPreview = False
                        .SendNotifyOnSendFailedWillRetry = False
                        .SendNotifyOnSendFailure = False
                        .SendNotifyOnSendHeldForApproval = False
                        .SendNotifyOnSendingFirstTime = False
                        .SendNotifyOnSendFailure = False
                        .SendNotifyOnSendingPeriodically = False
                        .SendNotifyOnSentSuccessfully = False
                    End With

                    If dr.IsFaxToNameNull = False Then
                        .ToName = dr.FaxToName
                    End If
                    If dr.IsFaxFromNameNull = False Then
                        .FromName = dr.FaxFromName
                    End If
                    For Each tblDocumentsToIncludeRow In mr.GetChildRows("MessageDocumentsToInclude")
                        'This line attaches the document to the fax... we do not delete on sending
                        'because we may send to multiple recipients.
                        .Attachments.Add(tblDocumentsToIncludeRow.DocumentName, RFCOMAPILib.BoolType.False)
                        Thread.Sleep(cGeneralParametersRow.SleepMilisecondsAfterFaxing)
                    Next
                    .Send()
                    'We need to give the fax server time to pickup and convert the documents before we delete them...
                    Thread.Sleep(cGeneralParametersRow.SleepMilisecondsAfterFaxing)
                    RFXServ.CloseServer()
                End With
                Return TrueStart Free Trial