[popcornmix]

This is consistently repeatable on certain channels - would a log of a ''non-working'' channel change and a log of a ''working' channel change be at all helpful?

Possibly, but it depends what is in there. Certainly a log might indicate the problem (whereas no log will not indicate the problem).
If you can catch any bad behaviour from a recorded file that you can get to me, then there's a much better chance of a fix.

It's possible the next newclock4 build could help due to this.