Checking for changes hangs

I set up our build server 1-2 months ago. And it has been working fine until the last 1-2 weeks.
We use TFS and the build runners are sln2008. The user for connection to TFS is the same as I use from my local machine.

The build hangs when "Checking for changes". It does it both on a large project and a pretty small one. Both when running it manual, on a checkin triggered build and a nightly scheduled build.
The first time it happened, we just rebooted the build server, and it worked again. But only for a day or so.

When I run the build server on my own machine it works.

What should I do?