xflock4 keyboard shortcut issue

[Grant]

I'm having a problem with the intel video driver which I can work
around if I keep DPMS disabled.  xflock4 enables DPMS with this line:

xset dpms force off

so I changed my Ctrl+Alt+Del shortcut in xfce4 to 'xflock4 && xset
-dpms'.  This command works perfectly if I run it manually, but it
doesn't disable DPMS when executed as a keyboard shortcut.  Does
anyone know why this is happening?

- Grant