I've got a similar problem. these days my max uptime is a day or two, where I was in the habit of leaving this box going for a month or two between reboots (and even then the reboots were usually caused by me needing to check that everything started up correctly after some change to a service).
In this respect, I agree with the other posters; it is most likely to be a memory leak problem. In my case, I can see Opera eating more and more memory as time goes on. Just in case you think its specifically an Opera problem, I've also tried with konqueror and firefox and get the same results, it just takes a bit more time. Equally, akregator seems to have quite a memory footprint and that is probably making matters worse, but I have to read RSS feeds somehow.
In fact, if I use ksysguard I can actually see the memory in use growing, albeit quite slowly. Equally, once it has gone 'bad', I see lots of hard disk activity as it tries, unsuccessfully, to swap its way to freedom.
My guess is that somewhere in the underlying libraries I have a memory leak, and this is a library that has been upgraded in an update process. Now, if I knew which one...
In practise, I'll rebuild the box sometime soon.