Please answer the following questions:
* Does this problem persist with all operating systems?
* Does this problem effect all computers on your network or just the specific system?
* How are you connecting to the internet, wifi, 3g modem, wired...?

Try installing wicd network manager. It actually maintains connection, unlike the default network manager. You can find it in the Software center, or install it using the terminal. to install using terminal, type the following:

sudo apt-get install wicd

You will then be prompted for your root password, which you chose upon installing the operating system. Enter your password, and it should find the package for you. Once it finds the package, it will ask if you want to install it. To install, press the "y" key. Wicd will install, and be used automatically to manage your network connections. If you encounter any difficulties, let me know. (particularly if it says "package cannot be found." if that happens, you may not have the proper repositories listed)