What could cause wired internet latency to increase between wired users?

Question

I have 3 roommates (4 people total). Three users, including me, use wired internet. The fourth uses wireless internet (connected to our network). During the evenings, they complain of high network latency - specifically massive lag spikes - that ruins their gaming experience. At times, they have pulled out my Ethernet cable (without my permission) in attempting to lower their network latency. Although I'm not intentionally running any bandwidth-intensive applications (I'm programming, and frequently executing Git commit and pushes), they say that the internet is much faster and the problem is fixed when they pull out my Ethernet cable (which puts me back on wireless internet). It seems like when I'm on wireless internet, even though I continue running the same applications, they're no longer plagued by high network latency issues.

Things to note:

1. At least once a week, our internet becomes unbearably slow for no reason (15 seconds to load Google).
2. My internet usage consists of heavy web browsing (usually every other minute, 25+ tabs), Git commits, and Dropbox-like file syncing (Cubby). Occassionally I game too, but that can't be the issue because they complain of latency issues while I'm not gaming. Also, I have since exited Cubby because I suspected the file syncing might cause massive lag spikes - things haven't gotten better.
3. They only seem affected when gaming. They haven't complained of slow web browsing.
4. The last roommate permanently on wireless internet only surfs the web occasionally.
5. Our service (in the morning when everything's perfect):

What could be the explanation? What is responsible for the high network latency issues (or at least massive lag spikes)? This is a frustrating experience for both me and my roommates.

I have two hypotheses:

1. More people using the wired internet increases average network latency for all. I'm not sure why wireless internet users don't affect the network latency of wired users (at least in our case, when they pull out my Ethernet cable and put me on wireless internet, they say there's no more lag).
2. It depends on the time of day. In our neighborhood, more users are accessing the internet in the evenings, which might increase network latencies.

Answer 1

Try simply replacing the cable between your computer and the router. You should use a packet analyzer like Wireshark and take a look at the traffic between your PC and the router/switch. It wouldn't hurt to monitor their segments as well, but I'd start with yours.

If their opinion the lag goes away when you're on wireless is valid, there is likely a layer-1 or layer-2 problem on your segment.