Essentially, I have a system of AV-600 powerlines set up in my house. I purchased a V4 TL-WPA4420 KIT to add to the system, to solve the issue of poor WiFi connectivity. After a bunch of messing around, I realised that the WPA4420 causes ping spikes for the devices connceted to the AV-600s. I'll describe the system to help explain:

Connceted to the router is the inlcuded AV-600 from the WPA-4420 KIT. Connected to my PC is one of the AV-600s. Connected to my brother's PC is an AV-600 also. As soon as the WPA4420 is introduced to the network however, both mine and my brother's computers experience ping spikes. As soon as the WPA-4420 is removed, the issue goes also. I have tried connected my PC to the WPA-4420 instead of an AV-600, and the issue still persists.

Pinging my router's IP shows that the ping spikes are occuring between my computer and the router, suggesting further that IT IS an issue with the powerline network. Im stuck here scratching my head, because, without the WPA-4420, my family doens't get good WiFi signal throughout the house, but, with it, I suffer high ping constantly. Any help appreciated.