Why do new websites work perfect but recurrent displayed websites hardly work?

Why do new websites work perfect but recurrent displayed websites hardly work?

Most probably the hardware in my HP Compaq nx7300 is damaged so I'm just trying to get by with my laptop and Windows XP Pro SP3 until I buy a new laptop. The strange is that my laptop worked quite well when I used a lot of programs simultaneously and had a lot of browser tabs opened, and PDF-documents opened, and also had the laptop running day and night for several days without powering it off (because I was working on a translation project). After that, when I finally restarted it and begun to use only a few programs and had few internet tabs opened, I immediately started to get more problems with slowness.

Until I buy a new laptop, is there anything I can do so that I can use internet without the problems I have now: www.google.se and Hotmail hardly work. Works a little better with Experts Exchange, but it seems that the pattern is that websites I frequently visit are most problematic to visit now. I get the yellow warning triangle and error message that the web page can't be loaded. When I, for a change, tried to search on Yahoo instead of Google (never do that otherwise, this was the first time on 3 years), Yahoo initially gets displayed immediately without problems. After a while, after having visited Yahoo for a couple of times, I get the same problem as with Google and can't visit Yahoo anymore (get the yellow warning triangle).

Should I empty the cache? Or should I try something else?

But it must be something with certain websites, because I also very often visit www.gmx.com and that is never any problem, I can continue to go there now, site gets displayed immediately.

My webbrowser is Mozilla Firefox.