What browser are you using?

I have that issue with Firefox when it's been working for a while. Then, when I restart the browser, it's fine but slowly gets worse.

The cause (I've read the synopsis) is memory leaks and on Windows 2000 (I don't know about XP, but not on Vista) there seems to be threading issues.

What this comes down to is that a lot of stuff accumulates in memory managed by the browser and it slowly degrades performance until it's restarted. I believe an upcoming release solves much of this.

So, what browser is it?