Why mapped network drives on Windows 2003 Server keep disconnecting?

I have four Windows 2003 servers (Standard Edition R2 SP2 and Enterprise Edition R2 SP2) that I’m trying to perform backups to a mapped network drive. I’ve mapped the drives (Y: \\10.1.1.100\Backups) and find that after about 30 minutes they show disconnected. This of course causes the backups to fail. I can click on the network drive and it will open. I’ve tried Microsoft solution (http://support.microsoft.com/kb/297684) but the drives still show disconnected after about 30 minutes. I’ve even tried just increasing the time to 65535 and no change in the disconnect time. The solution worked on our Windows 2008 server but none of the 2003 servers. Has anyone else experienced this problem and found a resolution?