Its been a long time since we have been hands-on with 2x42 generation cameras, but from a quick bit of testing with a 2442 cube camera we have set up on our demo 7716 NVR I think the problem you are seeing is a camera hardware issue with this particular model not being able to stably compress and decompress H.264+ footage.
We switched our 2442 to H.264+ and we saw a similar issue to yours with the camera working most of the time but every so often when we came back to the live view there would be no image and we would have to go into full-screen mode or go out of the live view and back in again for the image to reappear. this cutting in and out was most pronounced when using 4MP resolution, when we dropped the resolution the stability of the image improved.
We also didn't see any issue with the recorded footage actually stopping at the same time it cut out in the live view as you had seen with your freezing camera, but this may be due to the greater processing capacity of our larger, higher spec 7716 NVR.
Unfortunately with those older cameras, I don't think there is going to be any way to run both cameras at full resolution with H.264+ compression and have a stable image. I think it may be the case that both the 2142 cameras and the 7604-E1 NVR were not designed to support H.264+ compression but had the feature added with a later firmware and so this may be why it runs ok with one camera using H.264+ but when you try and run both you go beyond what the NVR & cameras are capable of. I don't think you will encounter any issues with continuing to run the cameras in standard H.264 CODEC, even if you go up to the full 4 cameras at 4MP I don't think that would be an issue.
