String.getHashCode returns different values?

Hi,
I'm using a hashtable to buffer incoming messages from remote clients. However, I have found instances where the hash value of the key when being stored is different to that of the same string when retrieving. However, some instances of retrieving pairs is successful.
The 2 snippets below show the store and retrieval mechanisms. An example key to store would be "192.168.1.123:<07x> - this is an example that, when attempting to retreive, is shown in the locals window, but containsKey returns false.
Any pointers greatly appreciated.

 

Public Function getRecievedMessage(ByVal strIPAddress As String, ByVal strCbusMsgID As String) As String
Dim strKey As String = strIPAddress & ":" & strCbusMsgID 'Concatenate IP and ID to form key
Dim strCbusMsg As String
If Me.qCbusMessageList.Contains(strKey) Then
'The requested message has been received; return its value and delete from the collection
strCbusMsg = Me.qCbusMessageList.Item(strKey)
Me.qCbusMessageList.Remove(strKey)
Return strCbusMsg
Else
'The requested message has not been recieved
Return Nothing
End If
End Function
 
Private Sub listenCbusIn()
Dim strcCbusMsg As CbusMsgContainer
While True 'Loop continuously
strcCbusMsg = Me.uIn.recCbus()
If Not strcCbusMsg Is Nothing Then
'CBus Message received, add to list
Dim strIP As String = strcCbusMsg.getIp
Dim strID As String = strcCbusMsg.getId
Dim strKey As String = strIP & ":" & strID
Me.qCbusMessageList.Add(strcCbusMsg.getIp & ":" & strcCbusMsg.getId, strcCbusMsg.getMsg)
End If
End While
End Sub