maybe you could try running that clearmem proggy when it slows down like that? It's linked from the "speed" section of the main page... Not sure if that would make any difference tho, it's about all I can think of.
I had a problem where every time I would try and type in internet exporer 6, it would crash or take forever. Turned out that the macfee virus scanner somehow was conficting. Who know why, it was an older version. Anyway, I assume you are running XP, what if any virus scanner are you running. By the way it it is something along these lines, norton 2002, solved the problem and works great.
Have you done any of the microsquish IE updates? I did the cumulative update and the system started freezing up. I'm running XP Pro 2002 and I had to roll back the system to before I did the update. I've always had this problem when updating IE ever since my second machine with Win 95 on it. If the update doesn't take right it screws everything up. I just don't use IE any more Just Mozilla and Opera.
It is probable it's you cable connection. Your using so much bandwith playing on line. Then as soon as you disconnect or log off of your online game your bandwith drops. Now everyone else has it and you don't, so when you start surfing with IE their's not enough bandwith. Then it's chug, chug, chug.