Host disconnect after few minutes

Hi I use ESXi 5.1,everything is fine,  my win 7 machine success connect to the host on vSphere

but after few minutes the connection lost and the vSphere down

I get massage

vSphere Client could not connect to "MY_IP_server".

An unknown  connection error occurred. (The request failed because of a connection  failure. (Unable to connect to the remote server)).

after that I can't ping even from the console,

i must reboot the host to connect again to host.

but after few minutes the connection lost again

that's how ithappens all the time.

Thanks