Linksys WRT54G Router does not connect

Hello,

I hope someone can help me because I've spent at least 15-20 hours on the phone with both my internet provider and Linksys to no avail. I'm not an expert at all (rather the opposite to be honest) with computers and routers but I have learned a little bit through this whole ordeal. Basically, my internet on my desktop computer is fast and works perfectly when hard lined to the modem. But when I go wireless through the Linksys WRT54G Router, my desktop computer and laptop both have issues. They will connect just fine to a site like yahoo.com or facebook.com but when I try to access gmail.com or my yahoo mail and my university's home page, it won't go nearly as quick and most of the time won't even upload the page. Everything was working perfectly until about a couple weeks ago. I had my router for almost a year Linksys won't reissue me a new one because there is "some" connectivity) but I need to be able to access these other sites. If anyone has any advice or suggestions, please let me know. I would extremely appreciate it. Thank you so much in advance!