Cable and DSL modems occasionally need to be reinitialized..The modem may be causing your issue. The next time the problem happens, turn off the modem and the computer...Wait a couple of minutes, then restart the modem, let is reinitialize for a couple of minutes, then restart the computer.

Another thought, assuming you're using an ethernet card with the cable connection...try reinstalling your ethernet card driver OR check your ethernet card provider's website and download and install and updated version.

Hope this helps.

Grif