Intel WiFi Link 1000 BGN driver 18.104.22.168 - 2011/10/27
T series Lenovo's do not have this issue.
Using the Wireless Connection Status window I see the Associated MAC Address jumping to another address which has weaker signal and then back to the original which is strong. During such an event Outlook 2007 loose server connection, users complain etc etc.
This happens intermittantly and again definately on more than one x220 machine.