[Roy Dennis]

I see you have 2 computers listed in your signature, is the same thing happening on both machines ?

If so, do you use the same external cables and hardware to connect to the external ?

When you refer to your preview screen are you referring to the small screen that the default would be fixed in the top left hand window or the singers screen that would show on the external monitor ?
If you are referring to the singers screen, that one would expand to cover your main screen if you double click it, double clicking it again should shrink it back to where you placed it.

If you have an external monitor attached for your singers screen then double clicking it should send it to your external monitor.

Now if this is the screen you are referring to then I reckon you have something wrong with the connection to the external monitor and as it loses signal causes it to jump back to your computer in full screen mode.

So my next question is how are you connecting to the second monitor ?

Check the cable connections and if you are using some sort of converter box check that out or replace it.

One other thing you can try first is reset your screens to their default locations Tools/Options Reset displays button.