I recently installed Ubuntu on a machine that uses a wireless network adapter (an USB key to be precise). It's the Tenda W302U and while it works flawlessly on Windows, it acts really strange on Ubuntu. I can find all internet signals around and when I connect to my network it works fine ... for about 3 minutes. After that I have to unplug and plug back the device so I can use the internet for another short period of time. I've downloaded ndiswrapper and tried to install the driver that came with the device but it says that it's already installed. I'm pretty sure I've used the same device with older Ubuntu versions and it worked fine. (I'm not sure if I had to do anything to make it connect back then.)
How can I fix this problem?

PS: Sorry for my bad english and keep in mind that I'm an absolute beginner.