Does it have IR LEDs?
Does the downtime correspond with when you might expect those LEDs to illuminate e.g. dusk to dawn.
Do you have a power supply of sufficient capacity?
For example, some PTZs might be able to draw sufficient power for most functionality from an NVR's POE port (or low power POE injector), but the IR LEDs or the heater switching on will be too much for the power source, and the camera will appear to fail.