Is it saying 'video failure' in the NVR logs?
How frequent is 'intermittent'?
What's the network arrangement, eg gigabit backbone with NVR on it, cameras (how many) on 10/100 switch or router, what speed connection is the switch/router uplink?
Any heavy network users that might be capable of causing an overload or congestion?
Does a 'ping - t' against the NVR and cameras show any breaks or rising response times?