serious lagFollow

ok i'll admit i'm not the brightest when it comes to all this computer stuff, with that said, i need some help. i got the new metal gear the other day and i been having FFXI in my ps2 from when i had dial up, but now everything's cable. so i was like now i need a router. so my dad got a router from a friend of his at work, and i got everything running, but i'm getting serious lag. everything will be working fine and then it'll lag up for a couple minutes then pretty much just stop altogether. anyone know what the problem could be? the router is a linksys model BEFSR41. i think it's pretty old so i'm assuming the router is the problem, but i don't really know. anyway, if anyone knows what could be wrong or what i could do to fix it, it'd be appreciated. thanks