Wireless speeds can fluctuate depending on where you are in your abode. Also, external signals such as microwave ovens and cordless telephones can affect signal.
Your internet speed can change if you're getting a poor signal to your modem from your provider. Other factors external to your modem can also cause ups and downs (your signal is shared).
Finally, your modem might be malfunctioning but that's less frequent. Is it an old one? What type of service do you have?