Users disconnected from Unix servers

Hi,
Every time a client/server is running on the Unix machine, users that are already connected got disconnected and no users can telnet to that machine.
I increased the pty but no luck, I forced also the NIC to 100MBPS-FD instead of autonegotiation, still no luck.
Does anyone have an idea of what’s going on and any solutions?
Thank you

Rales