In case you still have the problem, I have just one suggestion. It's a real 'long shot' to be honest as I can't think of any other cause at the moment. Try going into Device Manager and viewing the properties of the network card. Select the 'Power Management' tab and un-check the option that allows Windows to switch off the network card as a power-saving measurement.
On a similar note, change the system's Power Management settings so that it doesn't go into standby at all. Reboot, log in, connect your web browser and email client and then leave it for an hour (or however long it should take to know if the problem was still apparent).
Let us know if that helps