If you are only using 1080p content then you have no need to move from 8-bit 422 - any increase in bit depth only pads, there is no video processing and changing colour decoding to 444 will have no impact since the source is 420 (in common with UHD BluRay) and again, there is no interpolation. On the face of it there's nothing wrong with your HDMI chain (although you don't quote cable lengths - I assume 1 - 1.5m) but it may just be incompatibility. It's certainly not firmware per se since no one else seems to have the problem The only time I've seen it, and this is with 1295 boxes, was on an old Denon X2000 (same stable as your Marantz) but that was only at 2160p 10-bit 444, I've certainly never seen it at 1080p. It can't possibly be cable stress at 1080p, a damp piece of string will pass that and higher bitrate files will make no difference - we're talking about HDMI transfer speeds in the order of Gbps, not video bit rate in the order of Mbps. Your best option would be to test it on different equipment and see if you can replicate the problem - do you have another TV to try or someone local who doesn't mind you fiddling about at the back of theirs? If you can replicate the problem on different equipment then odds on it's the box, and I would assume a fault. If you can't then it's your kit chain, but I'm kind of leaning towards a duff HDMI stage on the box.