Menu items sometimes disappear after clicking

[Meddysong]

There's a bit of a strange phenomenon on my site. If I click a drop-down menu, quite often the revealed series of items disappears before I can click it. It's particularly obvious if I move the mouse slowly but it's fairly regular even if I move the mouse immediately to the first item.

Here's an example where I move the mouse slowly on a default theme, followed by similar slow movement on IPS's site for comparison's sake:

It's not just with slow movement though. It happens often enough with speeding to the menu item that it can be quite annoying.

Here's my site if you need a look: https://esperanto.org.uk/forums/

Can anybody help explain what might be the cause?

[Aiwa]

Have you disabled 3rd party apps / plugins and used the Support tool (Something isn’t working) to rebuild the theme cache and see if it’s still an issue? 

If so, I’d suggest submitting a support ticket for IPS to look.  It looks like a JS conflict somewhere... I’m on my iPad and can’t look into it, thought.