Welcome to the club.
I also have a Sunfire 245 with similar problem.
Telnet clients "hangup" from time to time - during the hang-up period I can
still ping their devices from the Server as well as print to networked printers
if attched to their terminals. The "snoop" command does not log any data
sent from the telnet session ( does log the ping session ) - the telnet session may wake-up after a while - normally the client powers the unit off/on.
Some clients have also reported telnet time-outs - may only happen after
x hours - most clients are fairly busy so still need to test.
Strangely I do not have problems with sessions connected directly on the Lan , only remote clients.
To overcome the problem I have made the clients telnet to my original Data General Server , then automatically rlogin them onto the Sun server.
This is only a temp solution till we can resolve the problem.
We also have a problem with delays in printing at times - have converted to CUPS which appears to have helped but not totally resolved the problem.
I think the "hang-up" and printing delays are related to the same problem.

Sun support have not offered any solutions as yet.
Best of luck - doogel