Whenever I record any live BBC TV channel, the resulting .ts file is playable - but it frequently freezes for a second, and also skips forward.

This occurs in 2 environments (1) when recording with an Ubuntu server located in London, and (2) when recording with a Windows 7 PC which is tunnelled to the London server. Both are new installs of 2.91.

On the Windows 7 PC, iPlayer in Chrome plays perfectly.

In the debug log, I see thousands of "Invalid timestamps" errors, plus a few "skipping 1 segments ahead, expired from playlists". Are these normal?

If I lower the quality, say with --mode=hlslow2, then the problem disappears. Speed on the connection is 15 to 20Mb/s.

What's wrong here?

Sorry, but our practice is not to provide assistance for use outside the UK - not that it makes much difference in your case. Performance of iPlayer in browser is irrelevant - different system entirely. Connection speed is irrelevant if download speed and/or stream encoding on your system can't keep up. Because of all the invalid timestamp errors, the live stream unfortunately has to be re-encoded, which requires more cycles. Even with a robust setup in the UK, glitches may still appear since the origin stream may have problems. Bottom line: don't expect perfect live recordings.