Some forumites have experienced a problem where they repeatedly get logged off the site - even when they are actively posting and reading articles. I've had to the extent that I repeatedly log in, immediately click on Board index and discover that I've already been logged off.
The first thing that you should try is deleting your cookies.
Try this first!If that doesn't work, then try the advice in this tut, and hard code your proxy server. It worked for me!
A "proxy server" (sometimes also called a cache) basically acts as a broker between you and the internet. ISPs run them to save bandwidth by serving up "cached" content and to make web-surfing faster.
Hard-coding the cache is not an ideal solution from a technical point of view, because once you've hard-coded this, if your ISP changes the name of their proxy or if the particular one you've hard-coded is down for maintenance, then you'll need to change this setting manually. Usually the aim is to make the proxying and caching "transparent" to the end-user. This is a workaround!
Note: This is possibly of more relevance to Saffies - with all the bandwidth in some countries, I'm not sure whether ISPs in more fortunate countries (fortunate bandwidth-wise

) use caches.
Thanks to bahamut for the initial suggestion and links!