I had a problem with some sourceforge mirrors timing out.
looking at /usr/portage/profiles/thirdpartymirrors
idsoftware ftp://ftp.idsoftware.com/idstuff ftp://dl.xs4all.nl/pub/mirror/idsoftware/idstuff ftp://ftp.fu-berlin.de/pc/g$
Theres a snippet
I just dumped the lagging mirror from the file and now i dont have to wait for the server to timeout before it jumps to the next