MikroTik

hi all
i have a very strange problem in my server

first : -

some clients have no ping to gateway and dns and other have normally and when i restart the server they have net normally

second

when the user working on internet normally he loged out the the login page come to him and when he try to login the server refused to accept the user name and password and after a while he accepted it and after a while he loged him out again

i try to solve this problem

i changed the lan card and the cables and the same problem

please tell me how to solve this problem

thanks