Small Business Server 2003 internet access performance degrades over time

I have recently intalled a new instance of Small Business Server 2003 for a very small network - the server and 5 XP (SP1) workstations, two on ethernet and three on Linksys 2.4 Ghz Wi-Fi.  For the first 24 hours or so all works fine, access to the internet from all workstations is robust and clean as well as access to the exchange server.  Seemingly randomely accesss to the internet becomes sluggish and on day three it becomes inoperable.  This is occuring primarily on the wireless computers.  Also access to the exchange server degrades (for all workstations) and on day three (or there abouts) becomes inaccesible.  I have rebuilt the server three times and the same series of events occures as described above.  I have stripped all security form the access points.   The server installation is out of the box, no tweaks.  HELP!!