[popcornmix]

Enabled "sync playback to display" via GUI Player Settings -> Videos Then ran 10 minute soak test with same BBC Catch-up item at same stream level of 3.1Mbps and lo and behold

• start skip = 6, after 10 mins skip = 6
  

In general "sync playback to display" is recommended to be enabled if you want perfect video.
It should be disabled (or you should use "PLL adjustment" on Pi) if you want perfect audio.

I suspect that these streams have an issue with audio/video timestamps.
Suppose video is 50Hz and audio is 48kHz, but due to separate processing of the audio and video streams, when they are combined,
you actually find in each second there are 50 video frames and 47952 audio samples.

"Sync playback to display" disabled means the audio drives the clock, but that is 0.1% slower than it should be (compared to video).
So, every 1000 video frames (20 seconds), there is an extra one which gets skipped.

With "sync playback to display" enabled, that means the video drives the clock and the audio is resampled (or PLL adjusted) to fit in
and you get no video skips.