In one rooom I have one PC with a router next to it. It's connected via LAN. Then I have another PC in another room. The distance between a router and the 3nd pc (in a straight line) is about 6 meters. That room is on the adjacent wall. Up until now I've been using a Strong's AP300 wifi repeater but I get high oscillations in download speeds and ping. Other than directly connecting my 2nd PC to the router via LAN, are there any other solutions you would recommend? Below is a drawing of how my situation looks like. I would try to avoid connecting it via cable as then i would have to drill through multiple walls.
Powerline is hit or miss.
Either it will work well, or it won't work well at all.
For folks that can't escape Powerline, I typically steer them to units that utilize the newer G.hn technology, such as the Zyxel PLA6456
If the room with the router and the room with your PC are on the same circuit breaker, you'll probably exceed 400Mb speeds.
I'd be way more prone to just drilling the 1/2" holes through the two walls just above baseboard level, and running a cable. No plug-and-pray, and no doubts about whether you're getting the best connectivity to your router.