Unable to cast COM object of type 'System.__ComObject' to interface type 'mshtml.HTMLDocument'

trying to attach to an IE browser.. it works for awhile then starts throwing this error...

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Unable to cast COM object of type 'System.__ComObject' to interface type 'mshtml.HTMLDocument'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{3050F55F-98B5-11CF-BB82-00AA00BDCE0B}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).
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OK   
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this code is fired on a timer of 500 millisecods...

could someone tell me how to fix it?? what I'm doing wrong?? || how I can improve it... My code is as follows...

 try
            {
                ShellWindows shellWindows = new ShellWindows();

                foreach (InternetExplorer ie in shellWindows)
                {
                    HTMLDocument document = ie.Document;

                    if (ie.LocationURL.Contains(ReserveRoomRsa + " ") || ie.LocationURL.Contains(ReserveRoomWww + " "))
                    {
                        if (bCodeScannerConnected && !idScannerShown)
                        {
                            idScannerShown = true;
                            ScanID(ie, "", "", "", "", "", "");
                            ie.Document.All.Item("guestHomePhone").Focus();
                            timer3.Enabled = true;
                        }
                    }
                    else if (ie.LocationURL.Contains(ReserveRoomRsa) || ie.LocationURL.Contains(ReserveRoomWww))
                    {
                        if (bCodeScannerConnected && !idScannerShown)
                        {
                            idScannerShown = true;
                            ScanID(ie, "lastName", "firstName", "homeAddressOne", "homeZip", "homeCity", "homeState");
                            ie.Document.All.Item("guestHomePhone").Focus();
                            timer3.Enabled = true;
                        }

                        DoNotRentAlert(ie.Document);
                    }
                    else if (ie.LocationURL.Contains(FindReservationRsa) || ie.LocationURL.Contains(FindReservationWww))
                    {
                        DoNotRentAlert(ie.Document);
                    }
                }
            }
            catch (Exception err)
            {
                if (i == 0)
                {
                    i += 1;
                    MessageBox.Show(err.Message);
                }
            }

NOTE TO SELF: Why do I need more details?? the Source Code is WAY MORE DETAILS than a PROGRAMMER Wants to give out when writing the next million dollar app!