It could well be proxies that are causing your problem. I take it you know what a proxy server is, so try disabling it in your browser. Also check that the refresh options related to proxies only check the proxy the first time you access a page.
Otherwise, you'll keep getting the same pages over and over again until your proxy server refreshes its copy.
Although Pprune is popular (and no offence to Danny here !), it is unlikely to be in the top 500 or so of most web servers and therefore, will not be cached. If it had more porn content, I'm quite sure we could get it in most proxy servers, thus speeding the whole thing up quite nicely.
The other problem could be with cookies. (We don't half make up some stupid words in IT !). Check they are on, otherwise pprune has no way of knowing when you last checked it.
Try clearing your disk cache as well - if it's full, and doesn't clear automatically, you might well be pulling cached pages out.
Can't offer any specific advice as there are so many different versions of different browsers and they all work slightly differently.
Mess about with your options or preferences related to caching/proxies and see if it clears up.
Cheers