Intermittent Network Connection

Hi All,
I administer a mid-size office with about 25 Macs, most running Panther. For several months now, we’ve been having infrequent network failures with symptoms ranging from problems printing, lost internet, lost mail, and problems connecting to the Xserve(which is running Jaguar server and providing sendmail, sharing files, running Pos/Oe),
Here is a description of the latest issue this morning :

Mail wouldn’t go out and internet not working; I unplugged the router, then all OK…until awhile later (employee) hadn’t received an email from (owner). It turns out (owner) and (employee’s) Local mail boxes had triangles next to them (weren’t working – wouldn’t go back on-line). I went downstairs and restarted the Server (which [owner] had done earlier). Local mail OK then.

This seems to be getting more frequent since switching to Panther. Also, we’ve recently added a few more flat-panel Imacs.
Internet is provided by cable modem. We’ve tried both Macsense and Netgear Routers. We’ve also replaced our switch.

The only thing I can think of now is something I just read about “Turning off Spanning Tree” on the switch. http://docs.info.apple.com/article.html?artnum=30922
It describes one of the symptoms we have in that two client machines sometimes get their addresses inexplicably switched. for instance, if you make an alias to one machine, you’ll get the other when you use it. Weird.
Can anyone help shed some light on this?
Thanks!