I've had a similar issue with 1608's on release 8.1.67, 8.1.69, 8.1.73 with a customer. They had 6 1608's on their network, and they would reboot every 5-10 minutes or so. This has happened with 2 separate customers. Tried replacing the switch, cabling, clearing and resetting the phones, the power supply, static ip or DHCP, connected them via POE or the local power supply. I didn't feel like learning how to wireshark and send packet traces so after replacing the 1608 with a 9608 and the 9608 never rebooted once ever... Bring the same 1608 phone on my in house network and the phone was fine....?!?!? I don't trust the 16xx series after this.