Wireless internet connection problems?

[Pengu45]

Hey guys Im currently trying to help my little brother who has had really bad internet connection issues for the past few months. For some reason his internet will randomly become really slow for a few days and then one day it will be fast, only to suddenly become slow again. Also, he used to be able to play world of warcraft just fine with no lag at all until it randomly started to have horrific lag and disconnects. I even installed WoW on my macbook and it ran just fine on my wifi connection. He connects to our home computer with a cisco linksys wireless router and under the devices section it says 802.11 g WLAN. It also says he has 54 mbps at the moment. I apologize if I sound like a computer noob but im not very experienced when it comes to internet connectivity, especially on a pc. Any help would be appreciated thanks guys.