Playback timing errors

Playing a course in HTML5 in either Chrome desktop, or iPad/Safari, we get a few (random) slides in the course which behave as follows:

- At some point, an animation such as a fly-in will clearly come to a crawl. Audio playback continues at normal speed.

- The audio will proceed, as normal, but all animations on-screen slow seriously.

- At the end of the audio track, there may be as much as 15 to 20 seconds of animations continuing on, in silence, until the last animation finishes.

- Maybe 3 or 4 slides out of 60 will perform this way. There's no way to reproduce the issue, changing the browser will resolve the issue.  I'm confident that it isn't reproducible on demand.

- The timelines and animations are perfectly aligned in Storyline 360. Slide Preview plays perfectly.

- Maybe unrelated, but the JavaScript errors that fault out of app.min.js are there as always.

- This isn't desktop playback. It's playback from a server.

Wondering if anyone else has observed such behavior in HTML5 playback.

Sam