The problem I found with iSpy, is that the 'older' cameras can only do jpeg-streaming, which is much lower quality than the 'native' mjpeg.
I am assuming both cameras are connected running wireless? You might check your local Wifi 'crowding' if connecting wireless. Could be somebody nearby is using same frequency-range/spectrum. Take a wifi-box and check what's going on near you (Windows INSSIDER, Android Wifi Analyzer, ...). You might find your preferred channel is near/on used channels in your neighbourhood.
Especially also check the final location where you want to put the camera.
I noticed on my camera's that they should not be too near to each other, as they interfere with each other.