Without a lot more information from you all anyone can do is guess. Start with the log files after a hang.
Rather than use a mains timer (that will cause SD card corruption as the OS won't be properly shutdown) I'd use a combination of a cron job to reboot each pi once a day and enabling the hardware watchdog to force a reset if the system hangs.
To reboot at 04:30 every day I'd add the following to root's crontab:
For more information on cron, run
It's been a while since I've used the hardware watchdog so I'm afraid you'll have to hit a web/forum search for that.
Attempts to contact me outside of these forums will be ignored unless signed in triplicate, sent in, sent back, queried, lost, found, subjected to public enquiry, lost again, and finally buried in soft peat for three months and recycled as firelighters