Shell Programming and Scripting

Hey, this time, I want my rhythmbox to play a different playlist at a set time (about sleeping time). my shellcode works, since if i run it it works, but when i make crontab run it, nothing happens. I try appending commands to the code, such as cp a file, and that occurs, so I have no idea what is wrong. Any ideas? sorry if this is a newbie question, but I really want to customise my system (to tell me to sleep ^_^)

Code:

#kill current rhythmbox
if ps ax | grep -v grep | grep "rhythmbox" > /dev/null
then
	/usr/bin/rhythmbox --quit
	sleep 2
fi

#load second profile - sleep profile
/usr/bin/rhythmbox --rhythmdb-file=/home/david/.gnome2/rhythmbox/rhythmdb2.xml --toggle-hide &

sleep 2

#play from that profile
/usr/bin/rhythmbox --play &

I recently found out how to redirect standard error - and it says that:

(rhythmbox:28057): Gtk-WARNING **: cannot open display:

so obviously I need the program to do its job without using the display. I think iw ill ask the rhythmbox forums (if there is one). thanks