i installed Ubuntu 12.04 amd64 today. And i am having a problem with my internet connection. Here is the situation, i am using an Alfa AWUS036H as my network adapter and when i connect to a wireless network it works only for a few seconds, and then the download rate becomes very slow and after a few seconds more it becomes zero. But the signal strength is always 3/4. Tried to disable IPv6 and it didn't help. I am new to Linux and ubuntu so i would be very happy if someone could help me.
I had the same issue with a Zyxel Adapter. After a few days trying to solve the issue without success, I bought a new adapter compatible "out-of-the-box".
If you are persevering enough, you will probably find a solution from someone in this community with your current adapter.
But if you're looking for a quick solution, check at this link :https://help.ubuntu.com/community/WifiDocs/WirelessCardsSupported which adapter is compatible (you can find some under 20$) and you will have quickly a working solution !