> Doing a "top" shows that Phex at this stage isn't hogging the CPU or
> anything, it's just sitting there looking at me completely frozen - a
> minimise/maximise and the window is completely blank/grey, showing
> that the program's not updating the screen.

Good info. To track this down: Start phex from a terminal. If it frezes again look for java errors. Mail these to werth@users.sourceforge.net.
Unfortunately I can't reproduce these lockups here and have never expierienced something like this so the errors would be very helpfull. If there are no errors at lockup and no cpu usage that would be strange.
Usually in swing apps, there are seperate tasks for updating the screen.
These call phex methods to see what they should update. I guess that these have a bug and crash, killing the swing update process by the way.
And please state what java version You are using when reporting a problem.
I made some tests with suns java 1.3.1, 1.3.0_02 and 1.2.2.
Just report the version displayed by the command "java -version"
Alexander