connection with the application object server could not be established

Hello Friends,,

we having 2 instance of ax one is for test another is for live..in test server only we having domain and DNS. live server is working fine, some times in test instance getting this error while login connection with the application object server could not be established,if we restart the system or unplug and plug the network cable its working again,this error getting only for few clients .. other client users are working fine..i tried to check network cable and other things also all are working fine ..need the solution for this