First at all, what about your signal strength?? Are getting a good signal level??
Also you have to put attention about the distance between the wireless access point and your computer, maybe you move the computer arround, and some access points may vary the connection speed according to the distance.
And my last recomendation, be sure there is nothing causing some kind of interefence between the computer and the access point. Some wireless phones, microwaves ovens could cause interference and reduce the signal strength.
P.D. Also try another browser.