Winterm client blue screen on Citrix XP FR3

[ChipsAhoy]

We are running Citrix XP FR3 on Windows 2003 server. We are running Winterms (with Windows CE) for clients. Approximately 1-2 times per week, a client will stop entering data on the winterm for approx. 2-3 minutes. When they start to enter data again, the winterm is blue screened - almost like a screensaver is on. However, there is no box to input your username or password and CTL=ALT=DEL does not get a responce. No error messages on the server. The only way to reconnect is to delete the session, at the server, and then let the client log back on. Any ideas?