It IS the network card
I was checking Google for a similiar case to what I was experiencing, and I stumbled across this post, which I see is actually a little old. But just to clear something up, here is my story.
I run a motherboard with an intergrated Realtek 8139D based network card. This has always served me very nicely, unfortunately, I came home one day to find the network not working anymore, I suspect it was becuase of lightning. Any rate I had a CNET PRO 200 network card lying around and plugged it in. Everything was fine, except from when I decied to play some Diablo II, it was aweful, was sort of stuttery.
Knowing that only my network card had changed as of late, I tried disabling it, and insantly my Diablo II returned back to normal.
I KNOW this is not a worm or virus, because I use an ISDN dial-up, and I have no stuttery game play while that is active, so I guess it is just the cheap CNET card. I guess that they took shortcuts on its controller chip, and used my system CPU to do certain tasks, which just slows down everything, especially intensive realtime stuff such as games.
Any rate, I plan on buying a 10/100/1000 NIC in a month or so, get me a good quality Intel or 3com.
Let me know if this makes sense to you AsianBoy.