Are you launching nm-applet from anywhere else (.xinitrc, xfce application autostart, etc.)? That would cause the issue you are having.
So I take it you have verified 'network-manager-applet' has been installed properly (including dependencies)? I'm pretty sure KDE has it's own plasma applet, but for the others this package must be installed for the applet to work properly.
It's quite possible the issue is hardware related (overheating, bad RAM, bad HDD, etc.). My suggestion would be look at sensor output, and check RAM using something like Memtest86+. This might help a bit as well: https://wiki.archlinux.org/index.php/Stress_Test