Internet not working properly on MacBook only

I'm using my sisters MacBook and this thing stinks! We got it in July of 2010, and it's always had this problem with the internet.

Basically, when I click on a webpage to open, it usually sits there doing nothing for about 2-3 minutes. Then it sometimes loads the page, sometimes it says "This Webpage Is Not Available." or just loads a blank white page.

The internet connection has a full 4 bars, and it works perfectly fine on every other internet-connecting device on the same internet connection.

We got it from Best Buy, and we had insurance on it but it expired. When we still had insurance we sent it in to be checked out. You know what Best Buy did? NOTHING. They kept it for 2 weeks and didn't fix a thing. I wouldn't be surprised if they just kept it sitting in the back of the store for 2 weeks.

We do not have the money right now to send it to Apple to be fixed, so I was hoping maybe there was a do-it-yourself fix for it?

Oh and sometimes it's up and down... like sometimes it loads perfectly like it should for a few minutes about every 10 minutes and then doesn't work again.

Does anyone else have this problem? :\