Disconnected by 2003 Server

I have a VB6 application loaded on 2003 Server. This
server is used as an application server and Terminal
Serve. The application is accessed by client through
Terminal Services client via RDP client. Sometimes, the
client sees the screen freezing up. At this point, from
server's task manager, the user's status
shows 'Disconnected'. This could happen on any button
click on the client side. I have about 20 concurrent
users, it just randomly happened on them. The same
settings were previousely used on Citrix server which
worked fine. Please help. Thanks, Fang