Windows 7 Pro X64 Group Policy Error - Mapped Drives Not Connectable After A While

Our domain controller is a Win Server 2003 R3, and we recently replaced several workstations with newer hardware running Win 7 Pro x64. For the most part, every thing runs smoothly. We have a shared folder on the 2003 machine that everyone uses to store their data. Everyone logs on to the domain, not the local machine. The problem happens when a machine is left on for more than one day. I don't know if it is on a regular interval, but I haven't noticed it as such. Eventually there is an error when trying to connect to the mapped drive: "U:\ is not accessible. An unexpected network error occurred."

When trying to browse the server from the Network in the Start Menu I get this: "Windows cannot access \\servername Check the spelling of the name... Error code 0x80070035 the network path was not found." I get a positive response when I ping the server's name and IP address from the client computer, and vice versa. I checked that the following services were running on both machines: workstation, server, tcp/ip netbios helper, DHCP client, DNS Client. The client computer's Ethernet adapter settings uses the server as the Preferred DNS Server in the IPv4 properties [it is set manually].

I tried ipconfig /flushdns on the client, then ran gpupdate in an administrator cmd window, still got an error. I ran gpresult /h gpreport.html and will attach that file for inspection. There are some errors in the system event log with GroupPolicy as the source and 1030 as the Event ID. Sometimes there is an error in the log with NETLOGON as the source and 5719 as the Event ID. What really makes it aggravating is that it only happens when the client computers are left on for more than a work day. If they are rebooted, it works fine.