DNS errors

Hi
we have a user came from other domain try to connect internet. I do not want to join that computer to our domain. what i did just connected to our network.It assigns the all( IP )network settings. It connects to the internet.
But it disconnects every 15 min. I try to put static IP is the same thing. I release dns and renew dns same thing.
i try to ping to the server iP no results, it is disconnecting from the server. Now my question is how to make it permanent to the internet.
Any suggestion -Please help me asap.
thanks