If you've noticed that your internet connection speed is slowing down to a crawl more often, then there are things you can do to diagnose and fix the problem:

• Make sure that you are really having speed issues
  A good way to test your connection speed is by using Speedtest.net or the Speedtest app for iOS or Android.
• Restart your router and modem.
  Unplug your router, then the modem. Wait about 30 seconds, plug in the modem, and then the router.
• Connect your computer directly to the modem.
  If your internet speeds are drastically different when plugged directly into the modem, then you know the router is the problem. If you still have speed issues, then it's likely the modem or the ISP.
• Install DDWRT or Tomato firmware on your router.
  These free firmware replacements for routers can greatly improve the performance of your router. Watch this episode of Know How to learn how to do this. If this doesn't make a difference, it may be time to get a new router altogether.
• Check your DNS performance, and use a different one.
  There is a free DNS tester called "DNS Benchmark" from grc.com. This will test to see what DNS will be faster for you. OpenDNS or Google DNS are both good, free options to use.
• Call your ISP and have them replace the modem.
  If you're renting a modem from your internet service provider, there's a chance it's older and not keeping up. They should replace it if you continue to have problems.
• Buy your own modem.
  If you keep having problems with the modem you're renting, you can get your own. In fact, you may even save yourself some money by buying it instead of paying monthly to rent from the ISP. Make sure to get one that supports DOCSYS 3.0 to get better performance. Call your ISP to make sure they're sending you DOCSYS 3.0 as well.