Maybe you have a dodgy router in the fleet, or a memory leak on one of them. This little snippet as bought me some time in some situations. Scheduling a reload is very simple. The below will reload the router every night at midnight.
event manager applet daily-reload
event timer cron name daily-reload cron-entry "0 0 * * *"
action 1.0 syslog msg "Automated Reload Triggered"
action 1.1 reload