There could be several possibilities for your internet connection to slow, or stop functioning. Here is a small list of things to check:
1) Run Spybot Search & destroy
This is a program that will help you find, and eliminate malware, it's free from Here
. It also helps speed your PC up by removing unwanted malware that can drain your system's resources.
2) Check your scheduled tasks to determine nothing is set to run during these " down " times.
3) Check to make sure any wireless connections are secured using WEP or other such wifi security. It's a little known fact that people will " pigy-back " on someone's wifi connection to use the internet and download. this can cause internet instability.
4) Call your Internet Service Provider. Your ISP can usually help you determine if the problem is with your computer or their connection.
5) Check your network settings. Your ISP can help you determine the best settings for your connection, as well you may check to see if the network device has an updated driver from the manufacturer.
6) If all else fails, you may wish to take your PC to a licensed technician, or replace the network device if you feel it is faulty.
In my experience most internet instability that happens on a regular basis during a set amount of time is due to malware, or scheduled downloads.