Don't know if this will apply in your situation, but I solved problems
with my iMac (running 8.6) intermittently disconnecting from my Win2003
Server. I was using my iMac (with files residing on the server) at the
time it disconnected - so I knew it had nothing to do with Energy Saver
settings or network idle timeouts. The system event log on the W2K3
server showed Event ID 12061 "Session from user "<my username>" was
timed out and disconnected by the server. The IP address of the
Macintosh workstation is in the data." On the iMac the AppleShare
Server Messages contained an entry "<My Window's Server Name> (My
server's IP address> via TCP/IP. The file server's connection has
unexpectedly closed down [<time> on <date>]".
Mistakenly I was thinking that the server was timing the iMac out
because of inactivity/idleness because of threads I had read on
similiar topics. But after researching more I discovered that the
server can also disconnect a client if it fails to respond in a timely
fashion. I traced the problem down to a "Duplexer" extension that
Apple released that allows one to manually set their Mac's ethernet
speed & duplex manually (essentially disabling the built-in
auto-negotiation feature on Macs). I had installed the Duplexer
extension on my iMac many months ago when my server was runnning NT4.
Recently I upgraded my Windows server to 2003, after which this Mac
disconnect problem started. I solved the problem by simply removing
the Duplexer extension from the System\Extensions folder. Restarted
and now iMac stays connected...
My guess is that in newer Apple OSs, code equivalent to this Duplexer
tool is built into the Mac OS, causing disconnects. Don't know how you
would disable this in newer Mac OSs... So I guess I shall consider
myself lucky to still be on 8.6