You can also check your bandwidth to make sure that you are getting what you are paying for. I know personally that sometimes my bandwidth would drop below what I was guaranteed to be getting. In which case you would need to call you ISP and try and resolve the issue.
But if you are using cable then it could just be a lot of people using the line. The way cable works is everyone shares the same line, so if there a bunch of people accessing the line in your neighborhood at the same time you will see drops in your bandwidth.