[menakite]

I have a guess what it may be. Can you try setting "sync playback to display"

This was already enabled

and A/V sync to resample. Any different?

while this was set to "Adjust PLL". It didn't have any effect.

Also can you try (in config.txt)
hdmi_clock_change_limit=50

This instead seems to have cured it. Changed A/V sync back to adjust PLL and still it seems to have worked - thanks!

It's hard to say for sure that something random is not happening though. I'll report back in a couple days.

I'm curious now - based on the name, I'd say that it limits the HDMI clock changes, and I assume that 50 is smaller than the default. Was the default value increased in recent firmwares? Also, why does this only affect a certain frame rate?