The origional stuffplug (not stuffplug NG) had a tag that would read a random line from a file. So to set a random line as your psm you would also need stuffplug ng as this has a command to set the psm.
Then in a convosation window just type:
/xpsm (!XRNDLINE<file>)
with <file> beining the name of the file you want the line from.
You can get stuffplug ng from
http://www.stuffplug.com
but stuffplug appears to have been removed from the plugins database, so ill try and find it somewhere for you.