Since the original post I have reinstalled Win 10, MC and also the Silicon Dust software. I only installed whats needed for its HTPC role and interestingly it can now manage ROHQ where before it did have all kinds of glitches. The Jmark score has also gone up so there may have been some driver issues or something. I have monitored the hardware when running tv with ROHQ and it certainly is pushing it hard. The gpu is occasionally hitting 100% and the temperature for the cpu and gpu is around 77 c but it manages.
The other screen artifacts I reported have been resolved, that was the crop edges setting in MC.
However, the pixellation when using it to serve photos as a slide show is still present. There is little hit on the cpu when doing this and its sitting at less than 20% even when playing tv. I too thought the i3U was probably underpowered for the role but the fact that I can play multiple bluray mkvs without a problem doesnt seem to support that. The problem seems to be the nature of the slide show, ie frequent burst of data required that seems to create the problem.
I have been looking at alternatives to the NUC, self build, ready build, other types of mini pc and nothing seems to fit the way the NUC does. I would have bought a NUC7i5BNH by now but there are problems with 4k playback (acknowledged by Intel) on all the 7 series NUCs and I'm waiting for Intel to come up with a fix. Its been ongoing for a couple of months and its not looking good for a fix. I already have ram and an M.2 ssd waiting!
In the meantime, Im having a new aerial fitted this week to make sure I have the best tv signal I can. There is room for a bit higher quality signal so the aerials going from the loft to outside and I will report if that makes any difference, but I dont expect it to address this particular issue.
I will try and record the pixellation effect and make the recording available.