My suggestion is to add sleep 120 at the start of this script. If it troubles upstart, then call another script with & and have that script sleep for a few minutes before setting the frequency.
That way, if you set a wrong parameter (there might even be changes - a voltage level that works in winter will not work on a hot summer day), you don't get a reboot loop, but have a small time to open an xterm and kill the script before it causes a reboot.