I'm not an overly excelling computer person but I do know some basic stuff so forgive me is some some of the stuff is off.
But over the last few months my brothers internet while he games is been kind of iffy and ping will sky rocket here and there for like 5min. He wants to blame my new computer, hand built by the both of us, saying that my computer is eating up all the bandwidth when my computer is doing nothing but just sitting there, not torrenting or anything. To me who is not even a big computer guy that sounds like a load of crap... cause I monitor the bandwidth my computer takes all the time 24/7 on my second screen and it only jumps when I torrent and I two sometime get the lag spikes but I just hop off games for the time. I think the problem is a new switch for the network he put downstairs with the modem, the switch was being tossed from his work and he thinks since he does a little I.T. work he knows damn near everything. And I'm inclined to think well if the place was tossing it out then it probably broke and malfunctioning in some way but he wont hear s*** about it.
For gaming especially you want a Gigabit port, I would check the switch port speed. Considering his work was tossing it, it's probably only 10/100 which is causing you the problems. Also what router are you currently using and did the problem exist prior to installing the switch?
It is pretty easy to see if it is something in your house outside. Run a continuous ping to the gateway address..ie your router.
When you see issues in the game check to see if you are getting any loss or large spikes in the ping times to the router.
A switch is a pretty stupid device it should cause no issues. Generally all switch can pass traffic on all ports at maximum speed up and down simultaneously. You might get loss if you exceed a ports bandwidth but they can't really delay traffic.
Need less to say I hope you need to run all these tests using cabled machines no wireless since wireless is known for completely random traffic issues.