loose computer on network after 5 hours

What i can't figure i have a network set up all machines are on same workgroup, same router yet after 5 hours i loose the same machine every time, it will not show up on workgroup till i restart the computer.

It has been reformatted twice but after 5 hours it drops off the workgroup but still has access to the internet??.

I can't find a reason for this and each computer has a unique name on the workgroup.

Resetting the router has no effect either i have to physically restart the computer that is on the network to get file shares to show up on the workgroup.

The firewall has been disabled on this computer as well so its not a firewall issue.