Unstable XP !

I've been running Win XP Home for several months and found it extremely stable and much-improved over Win 98. I recently installed a 80211.b WAP that wireless networks another desktop (Win 98) and a laptop (Win XP). My original desktop that has had the Win XP for several months is connected to the wireless router via an ethernet card & cable. It now seems to be acting rather strangely. One can do a fresh boot and everything works well - IE, MS Outlook, etc. all connect to internet, etc. After several hours (of little or no activity), the next time you try to use IE or Outlook, they fail to connect to the internet. There is an icon on the taskbar for the wireless router (Dell Truemobile 1184) that indicates the desktop is still connected to the router and the ISP. However, for some reason, instead of identifying the correct name of the wireless network, it will say "NULL" instead. I did try re-installing the XP software and it didn't seem to help. (I may have erred, but I think I chose the "upgrade" installation.)

Any ideas on what's going on?