No internet connection via Wi-Fi

I have a 2009 MacPro Quad-Core with Lion. It connects to my Wifi networks, but not to the internet. I called the provider, and they said they know the problem, made me set up a "guest"-network, then it worked for a day, and dropped again. Repeated process, same thing. It will only connect with a cable, then it's fine. But no Wi-Fi. Any ideas? I need to use the wireless connection...