To add on to what mike27oct is saying, if you live in a densely populated area, your neighbors could be causing some of your WiFi issues.
In the neighborhood where I used to live, all the houses are around 1200-1600 sq ft with each house on a 1/8 acre lot. And I didn't even have neighbors behind me because it's just undeveloped dirt. So I didn't think it was all that densely populated. But after using a WiFi analyzer app, I found out that the WiFi signal was very, very crowded. Not only were every neighbor's WiFi routers jamming everyone else's, there were dozens of WiFi printers, security cameras, doorbells, DVR's and every other kind of device you can imagine. Even if your router is not hopping channels and your WiFi signal is pretty solid today, your neighbor's router can hop right to your channel and diminish your signal the next day. If you live in an apartment building with dozens or hundreds of units, FORGET ABOUT IT! You might as well get WiFi extenders with hardwired Ethernet and put your Echo (or other device) right next to it.
As an example, let's say your router is on channel 6. Your neighbor happens to be on channel 6 as well. But for now, everything is working just fine since your devices are a lot closer to your router than your neighbor's router. When your neighbor comes home from work, he walks around the house with his iPhone. His iPhone is connected to channel 6 of his router. Everything is still fine at your house. At the end of the night, he puts his cell phone down next to the window to get a better cell signal and that window happens to be adjacent to your window. With his cell phone using channel 6 in close proximity to you, your channel 6 is now diminished. As you can see, this would cause intermittent problems with your WiFi. If your neighbors have lots of people in his house, every one of those people could be moving around with their mobile devices (phones, tablets, laptops, etc.) that can potentially diminish your WiFi signal. Also keep in mind that laptop and desktop computers tend to have stronger WiFi antennas that can affect you more negatively. But luckily most laptops/desktops will go into standby mode (sleep) when it's not in use. Again, this would cause intermittent problems for you.