internet availability

1) From a non tech managers perspective, the internet, i.e. staff accessing www.google.com which we access when connected to the corporate LAN is often hopelessly slow or sometimes not available for hours on end. As a general rule, will that be an internal issue or an external issue?

2) can you give me perhaps a top 3 reasons why internet access goes sluggish/off?

3) And could this be a result of poor internal admin/management, if so are there any prime candidates of what they are not doing thats leading to such issues? I.e. what are the internal admins requried to do on a daily/weekly/monthly basis to keep the internet connection for internal staff running smoothly?