Why is My PC's Wifi So Unstable Every Few Seconds?

I just got a new PC a week ago, and I've been experiencing some frustrating internet issues. Every 15 seconds or so, the connection gets super laggy, especially when I'm trying to play games—it becomes nearly unplayable. I'm using Wifi instead of a wired connection and, unfortunately, I can't switch to cable because of the layout of my apartment. My PC is located the farthest from the router, but interestingly, my old laptop had no such problems in the same spot. I'm really not sure what's causing this lag and have tried several things to fix it, but nothing has worked so far.