Is this happening to you on all servers, a few of them or only on one? If it's the last one it's probably a server-side thing, if it's option one or two hmm... check if you have a firewall on, it could disrupt your connection to the servers, could be also that you have a virus or something, check with your anti vir. Other than that i haven't a clue, too little input about the problem.

Sorry man, i'm no expert in net issues, i can only advise to do the standard things. Check if your firewall is on, check you comp for viral activity, reinstall quake, also you could download netlimiter and make sure there's no shit running in the backround that blocks your packet traffic. It also could be that you put some stuff in your config that makes the problem, move your q3config outside the folder and then run quake, check if the problem persists. Wish you the best of luck with solving this, man, it sucks real bad not being able to play quake, i know

try this /net_noudp 0

I have simmilar problems with some local servers on my wlan. My best bet is they are caused by wrong routes, and dns problems, cause ip's dont translate well. Dont know if it is your problem too.

Try what Obelisk said, and also check for spyware or virus. They sometimes takeover some of windows services. If it all fails, e mail your isp and ask if they changed someting on the network. Maybe they are fixing something?