strange telnet problem

[21cn <lemon_3 21cn com>]

hi:

i's runing redhat6.1 on my LAN server,recently,a very strange problem
occurs,that is: sometime i can telnet to server without any problem,but
will be broken automatically by server a few minutes later,sometime i
even can not telnet to server but at the same time some other clients
can telnet and broken a few minutes later,please tell me why?

thanks in advance