Missing and random notes in playback and uploaded score

This error is kind of hard to describe.

I have recently installed MuseScore Studio 4 with MuseSounds onto a new Windows PC, which is a little faster than my old PC and with more RAM.

New PC: i7-6500U CPU @ 2.50GHz + 16GB RAM (Windows 10)
Old PC: i5-3337U CPU @ 1.80GHz + 4GB RAM (Windows 10)

The old PC had dreadful (slows down, crackly, stuttery, garbled) playback, but when I uploaded a score, it played back well on the website.

The new PC seems to playback better - for the first few bars, and then random notes seem to play and voices disappear and reappear. When I uploaded the score, the same errors appear in playback on the website.

I've uninstalled and reinstalled MuseScore Studio and MuseSounds to no avail.

I'm guessing some setting(s) on my new PC must be causing the problems, but I'm at a loss as to what it could be.

Any ideas?