This sounds like a host/network or license issue. If you do not have a valid license for the emulation type you are running you will be disconnected after a certain amount of time and be forced to log back in. You can see which emulation types your device is licensed for by launching TE then selecting Options>About. If you are running in demo mode there should be a demo notice on connection as well.
If you are licensed for the emulation type and still have the issue I would recommend upgrading a terminal to the latest version of TE and see if the problem still exists. You can get the latest from our downloads site at : http://www.wavelink.com/Motorola%20(Symbol)-MC3100-Emulators-downloads
If you still have the issue after verifying licensing and attempting the latest version please create a support ticket and someone can assist you further.