Add the following line to your (desktop user, not root) crontab:

*/5 * * * * [ -z "`pidof -x /usr/bin/nagstamon`" ] && DISPLAY=:0.0 /usr/bin/nagstamon &

This will start nagstamon and make sure user doesn’t closes them. Tricky is configuration of connection to you Xserver / graphical environment. But solution is simple: set $DISPLAY environment variable to :0.0 value, which means ‚use the first desktop running on localhost‘.

Did you know ‚xauth list‘ command?