Windows XP port forwarding and Internet Connection sharing is very flaky. I had many of the same problems, where things would work for a bit, then stop working. Do yourself a favour. Invest in a hardware router that allows port forwarding. Right now you can get one at Future Shop for $115. This will give you a much better port forwarding solution while offering much better protection against intruders than Microsoft's solution. You should also notice a bit of a rise in line speed, since you're using hardware to share the connection and not the software ICS.
i had a quake3 rocket arena 3 server running on my other computer (192.168.0.2) go to ur connections properties then go to advanced and go to settings where is says services double click on Web server put in the IP (192.168.0.3) and click ok (i am assuming you want a web server running on ur client machine) if not just click on add. put in the ip. and fill in the port #'s