Download and install software inSSIDer (just google it).

Installed and see how much wifi network around your house, and try to change your wifi router channel to a channel that is less congested.

I'm facing the same issue previously when a new neighbour move in next door and having same channel as mine. The Wifi become very unstable especially at nite when they are active using it...

another possible issue that i found out is that if you added new device that using the same wireless freq 2.4GHz, it might cause interference....
Found out my new free wireless mouse actually cause interference when i do download that takes >5 minutes, the wifi transfer speed will drop and up again...
end up i throw away that free unbranded wireless mouse and use my old trusted atech mouse

hope this help...