There are few reasons for shutting down of internet. Your ethernet port may be damaged so that it can't receive packets properly. Or your router provided by your service provider may be damaged to give constant connection.
Following are the reasons if your interne keeps shutting down:
* There might be a problem in the phone line if you are using dsl broadband make sure line has no disturbances. *The servers of the company providing the service might be too busy or having some problem
There could be several reasons for that:
1) The most likely reason is that your computer is infected wiht a spyware or virus. Run your antivirus program to remove the infection. 1) If you have a dialup connection, you could have unknowingly selected the option that automatically disconnects your internet after a certain amount of time if it is idle.