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 that was the problem, I would expect the WiFi to be slow at all times, and not just sometimes. Also, I have a gut feeling that it might be a DNS issue (for example, getting a page from a known server is faster than accessing a new server), but I don't know how to tackle it.

Update: despite numerous updates in the meanwhile, I still observe this behavior. It happens always when I access my WiFi router at home after returning from work; when I reboot my laptop, the connection speed is good again.