Please check which FW are you running, from the device status menu. You might need to upgrade it.

I would say you have a flakey POE port or POE problem.

Try plugging in a power brick to the phone to see if it resolves the issue. Then back-track.

Another simple fix would be to switch ports where the phone is plugged into.

I have seen blades in 4xxx go on the fritz with POE problems.