Why is my internet fast, but slow sometimes?

Just a quick briefing, I have the internet speed up to 18 or 21 or something like that mp something or whatever the speed diagnosis is called, I have AT&T broadband? We have a modem and it's plugged into the phone cables in the wall, if that makes a difference, but most the time my internet is really fast, we have it password protected and there are only 2 computers in the house and 2 iPhone's, but the one computer and iPhone are hardly ever home, and don't use the internet much, and randomly, my internet will get speeds up to 0.62 (i check on speedtest.net) why does it do this? I watch a lot of tv/netflix on my computer so when it's slow like that, my shows pause and have bad quality all the time, it's quite annoying, but than after an hour or two it goes back to being fast again, it does this at the most random times, anybody know why? (We also never 'download' anything that would make it slow) please help!

It could be a variety of issues.

For one thing, it could be that your ISP (Internet service provider - which in this case is AT&T) might be experiencing congestion as a lot of people in your area who subscribe to AT&T internet access might be using the internet and using high-bandwidth applications. There isn't really much you can do about that.

However, it could very well be possible that your wireless router (the device that is giving you wireless internet access to the iPhones and computers) is old and needs to be replaced. New routers usually go for $40 and up depending on how serious you are about your router choice.
If you do decide to get a new router, make sure that it has 802.11 b/g/n or b/g/n/ac. These are the latest wireless standards that will allow you to get the best wireless speeds from your AT&T service.

The other piece of equipment that might need replacing would be the modem, which turns the cable connection into internet access. If you've never replaced it before and you've had AT&T internet access for a while, now might be a good time to replace your modem.
This is the modem that I use that I've had quite good experiences with: http://www.amazon.com/...004XC6GJ0/

Your ISP throttles big services like netflix and other similar companies so they do not hog up all of their bandwidth however recently these services are working deals with ISP's and paying them for usage so they can pass their data to the customers without being throttled. You are at the mercy of your ISP on this one.