Cisco spa508g reboots once an hour

just installed FreePBX 12.0.37 distro 6.12.65-24 on a customer server. the phones are spa508g. using the paid end point manager. everything boots and runs fine but each phone reboots once every 60 minutes. it is not that all the phones reboot at the same time. they reboot 60 minutes after the last time they were rebooted. the phones, pbx and router are connected to a single switch. my best guess is that this is occurring when the phone is re-registering because the phones are set to reregister every 3600 seconds. i can’t find any setting in the phone that could be causing it to reboot - any suggestions would be greatly appreciated.