Linksys BEFSR11, 41 and 81 routers seem to be causing this a great deal with the latest firmware version.....
If you have the BEFSR11 or 41 router, you need to run the 1.45.7 firmware to solve this problem.
You can download the known good Linksys 1.45.7 firmware for both the BEFSR11 and BEFSR41 V2 routers at
http://ftp://ftp.linksys.com/pub/befsr41/befsr_fw_1457.zipYou also need to go into the router and disable UPnP and QoS if it's there and you should ONLY have TCP/IP checked in the network properties for your Internet connection.
hope this helps el problemmo
-JxL