Speedtest results are not real useful metric when you are talking online games. A data transfer and most non game traffic easily tolerates a spike in the latency. Wifi for example would not exist if all traffic was online game traffic. Speedtest likely is seeing the spike but it has almost no effect on the amount of data that can be transferred. Really bad packet loss you might see the graph go down and then back up while speedtest is running but the average speed will cover that up.
Be very sure it is your poweline unit causing it. You need to leave a constant ping run to your router ip in back ground window. If you see spikes that correspond to your game then the powerline is the cause.
Unfortunately there is very little you can do about powerline issues. Powerline unlike wifi is pretty much immune to interference from outside your house. Still powerline is not that different than wifi. It is basically a radio signal that is using your power wires as a antenna. The signals only go though the wire not the air so only something on the wire can interfere. Problem is all the electrical outlets in your house are in some way connected together....that is why powerline can even work. Could be some device in another room causing it. I have a shop vac that I can plug in anyplace in the house and it will kill some powerline units I used to use.
Your only option really is to try to find the offending device and find a way to filter the offending data out. Sometimes a surge protector but that is not a option if say the device is a ceiling fan causing it.
In any case your first step is to be really sure it is the powerline, it is not a common thing. Only other technology would be something like moca.
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