Wireless Internet Stops Working / Attempts To Repair

Wireless internet connection stops working after anywhere from 10 minutes to two hours, a message pops up telling me it is "impossible to connect to my preferred wireless network", and then it fails to detect any wireless connections at all. The strangest thing is that when I try to "repair" the connection, which used to work, the computer immediately shuts down and restarts itself. It also seems like when I attempt to download something, like a movie for example, the connection fails right away, while if I'm just surfing the web it usually lasts around an hour before stopping. And when I restart the computer again it works perfectly fine at first, like right now, but eventually does the same thing.