More info on your network setup would help also:
- What make/model of NIC?
- Cable or DSL Internet access?
- Is there a router in the picture, or does the computer connect directly to your cable/dsl modem?
- Are you using DHCP or static IP assignment?
If you're using static IP addressing, check out some of the suggestions in this article:
http://www.stevewolfonline.com/Downloads/DMR/Doc/Linux/Network/NICConfig.txt