I acquired an early power line network kit to reach my printer at one end of the house, a few years ago. It was a Devolo kit with two transparent blue adapters saying 85Mbps on the outside.
Years went by and I needed to extend Wifi coverage to an upstairs bedroom, so bought a Netgear WiFi extender, with the words AV200 in the name. Sometimes this worked ok, but mainly it was a disaster and drove me mental, the connection to the external internet took a couple of minutes to stabilise, and was generally flaky.
I tried plugging the adapters directly into the wall, and tried other wall sockets. I investigated Wifi channels, I considered using a divining rod to find a good network signal, and finally considering burning the house down.
Then I came across this post: http://www.smallnetbuilder.com/basics/lanwan-basics/31238-slow-homeplug-five-ways-to-boost-powerline-network-speed
The magic answer is this: Don’t mix the 85Mbps equipment with the 200Mbps equipment, they don’t work on the same set of power wires, by removing the Devolo adapters the whole thing stabilised. I test the Netgear unit upstairs and got a solid 30Meg as reported by Speedtest, as I have fibre broadband from Plusnet.