1 - Disconnect all but one camera from the DVR. Check the image and repeat with each of the cameras. If each camera is fine when connected on its own then one of the cameras is introducing the fault on the other (as the shield of all the cables are effectively connected together by the recorder)
2 - You don't mention whether the cameras are 12V DC powered or PoC (Power over Coax) If they are 12V DC and share a power supply, connect each camera to the power supply, on it's own, in turn to see if the issue persists when they are isolated from one another. This should be done when the cameras are drawing maximum power (on a night with infra-red on) If the issue disappears, you may need additional power supplies. Some of the Hikvision data sheets for the HDTVi analogue cameras that you're using, do specify that you should use one power adaptor per camera.
3 - Check the connections. Check not only that they are made correctly and secure, but also that they are not touching anything connected to ground such as metalwork. I've seen earth faults cause issues like you have and the fix was as simple as taping the over the body of the BNC connection at the camera to insulate it from ground.