I have this same issue with my 5 camera + NVR setup. Its a DS-7608NI-K2 8 Port, 5 cameras (2x DS-2CD2142FWD-IS, 2x DS-2CD2155FWD-IS, 1x DS-2CD2442FWD-IWS) My cameras don't connect directly to the NVR, they are reached via the switch. The issue for me is worse on the 4MP 2142 models however...