Subtitles going out of sync

[SunnysideUK]

My usual approach of ad marking films and cropping out the adverts seem to have a variable effect of causing any subtitles going out of sync.
On play they seem OK but typically speed-up, displaying early, even within a minute or two of playback.
If I stop the playback and then resume the subtitles get back in sync again (displaying after a short delay), but than go out of sync again.

However, I have seen sync problems on just decoded recordings. I recorded Dune (1984) yesterday and this problem is prompting this thread.

What is the cause and is there a fix for this?

UPDATE.
OK, I thought this was custom firmware related so only look there, but I now realise this is a known Humax problem;
ref link: https://hummy.tv/forum/threads/subtitle-sync-problem.9479/

 

[Black Hole]

The crude cropping of TS streams that CF can accomplish fails to patch up subtitle stream timestamps, so yes you will get that problem with ad break cropping. If instead you just settle for bookmarking the end of ad breaks, a simple skip to next bookmark (second row up, third button from the left) skips ad breaks on playback with minimal intervention and avoids the problems of having cropped the TS.

However, yes, we know subtitling is troublesome in general, as standard.