SQL Server 2000 transaction completed error message

I am using ASP.NET / VB.NET to access SQL Server 2000. I am accessing an table to insert / update which is heavily hit during peak hours of usage. The following is the piece of code which is does not consistently gives me the error. I had used DataAccessApp block provided by Microsoft. The piece of code is :

 Public Function MakeOrder(ByRef OrderDetails As OrderInfo) As Boolean
            Dim myCon As New System.Data.SqlClient.SqlConnection(ConnectionStr)
            Dim myTrans As SqlClient.SqlTransaction

            Try
                myCon.Open()
                 Dim sProcname As String = "CreateNewOrder_sp"
                 myTrans = myCon.BeginTransaction("MakeOrder")
                paramOrderNo = New SqlParameter("@OUTI_ORDERNO", 0)
                paramOrderNo.Direction = ParameterDirection.Output
                ParamExtn = New SqlParameter("@OUTI_EXTENSION", 0)
                ParamExtn.Direction = ParameterDirection.Output

                SqlHelper.ExecuteNonQuery(myTrans, CommandType.StoredProcedure, sProcname, _
                    paramOrderNo, _
                    ParamExtn, _
                'Other Parameters
                    )

                ' Call the detail section inserts
                Dim DRow As DataRow
                sProcname = "CreateNewOrderItems_sp"

                For Each DRow In OrderDetails.DetailSection.Rows

                    SqlHelper.ExecuteNonQuery(myTrans, CommandType.StoredProcedure, sProcname, _
                        'Other Parameters
                        )

                Next

                ' Everything works fine, commit the changes.

                myTrans.Commit()
                Return True

            Catch Ex As Exception
                ' Rollback the transaction
                myTrans.Rollback("MakeOrder")
                Throw Ex
            Finally
                myCon.Close()
            End Try
Emd Function
======================================================
During Peak hours I get the following Error Message
=====================================================
Server Error in '/OrderEntry' Application
This SqlTransaction has completed; it is no longer usable.

Description:An unhandled excepion occured during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details:System.InvalidOperationException: This SqlTransaction has completed;it is no longer usable.

========================================================
The problem is this does not happen consistently and we are not able to identify what the problem is to fix. We were able to see a lot of locks during the process and some times few users blocked each other, but there was never a deadlock situation. Can anyone provide light on what is going wrong.
Thanks