Is it all of your phones or just one? If you download error log ALL and then click on tools and then ipkt reboot diagnostics, this will tell you when the phone goes offline, and may assist in how the phone keeps failing.

I had this happen once to an entire office where something was broadcasting out on the network. We were able to identify this with wireshark. If it is at a specific time every single day, I would check to see if the customer has a printer software on their PC that checks for maintenance on their all in ones. That occured several times for me.