WiFi slow sometimes, reboot helps, how do I debug it?

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http://askubuntu.com – Ubuntu 12.04.1 with all updates installed. Laptop Lenovo Thinkpad X230 with Intel Corporation Centrino Advanced-N 6205. WiFi sometimes becomes extremely slow. Often this occurs when I wake the system from suspend and connect to a different network. I find no obvious clues in system logs. /etc/init.d/network-manager restart doesn't help, but a reboot does. How can I go on with debugging this issue? In specific, which parts of the system should I try to restart (without a complete reboot)? I know of problems with Intel WiFi (see for example this question and the instructions here), but if tha (HowTos)