BBC1 HD recording glitch

I took it that he had 100% quality and was thus reporting signal strength - but it could have been clearer.
Thanks, Quality remained on 100% during the observation. This seems to be an intermittent fault and I still cannot reliably reproduce it, so I don't suppose a one-off observation will always catch it. I will certainly try the hidden menu though.
I've studied signal quality on both tuners. Doesn't seem to drop below 100%.
I can reproduce the glitching by recording on channel 35 (SD; 51%strength 100%quality) and simultaneously viewing BBC1HD (71%strength 100% qual)
I looked at the signal strength in both tuners at all the frequencies available during this test. No drops below 100% quality.
I also recorded channel 35 and BBC1HD at the same time. BBC1HD recording was glitchy.
Not really any closer to diagnosis. I'm feeling like I'll have to accept this situation or buy a new unit.