If I select text using the cursor keys everything works as expected. If I use the mouse the selection de-selects itself after a fraction of a second - long before you can do anything with it. All other mouse actions work normally.
I only have a few Qt based apps but it is only LyX that exhibits this behaviour. I am using LyX 1.5.5 on Debian Lenny with XFCE. Ideas anyone? Regards, Iain.