Hello @AdrianS01 

1.    TV going pixilated could be caused by low bandwidth, as device connected  to powerline and devices connected to router share the total bandwidth of router, was there any ( high traffic) network activities like streaming or file downloading on devices connected to TL-PA4010 at that time? 

2. To isolate the issue, please unplug the powerline kit for a day or two and see if you get Poor Wi-Fi

Try a different wall socket in a different wall socket, probably a socket a bit further away  from the router. 

3. Was the powerline work fine with the same router? Did you change any settings on Wi-Fi router that may cause the issue. 

For the poor Wi-Fi, was there any disconnection or Wi-Fi error or only TV was pixilated? You could test the Wi-Fi speed once plugging TL-PA4010 ( unplug the device in extender unit in case it shares bandwidth)and see if there is any difference

4. Can you know your network map, like Router Model XX- PA4010 main ___ PA4010 extender-PC

Router -TV.